Henrietta Swan Leavitt (updated)

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Henrietta Swan Leavitt
Henrietta Swan Leavitt
Born	July 4, 1868 Lancaster, Massachusetts, U.S.
Died	December 12, 1921 (aged 53) Cambridge, Massachusetts, U.S.
Residence	Cambridge, Massachusetts
Nationality	American
Citizenship	United States
Alma mater	Radcliffe College, Oberlin College
Known for	Leavitt's law: the period–luminosity relationship for Cepheid variables
Scientific career
Fields	Astronomy
Institutions	Harvard University

Henrietta Swan Leavitt was an American astronomer. A graduate of Radcliffe College, she worked at the Harvard College Observatory as a "computer", tasked with examining photographic plates in order to measure and catalog the brightness of stars. This work led her to discover the relation between the luminosity and the period of Cepheid variables. Though she received little recognition in her lifetime, Leavitt's discovery provided astronomers with the first "standard candle" with which to measure the distance to faraway galaxies. After her death, Edwin Hubble used Leavitt's luminosity–period relation, together with the galactic spectral shifts first measured by Vesto Slipher at Lowell Observatory, in order to establish that the universe is expanding.

Life

Early years and education

Henrietta Swan Leavitt was born in Lancaster, Massachusetts, the daughter of Congregational church minister George Roswell Leavitt and his wife Henrietta Swan Kendrick. She was a descendant of Deacon John Leavitt, an English Puritan tailor, who settled in the Massachusetts Bay Colony in the early seventeenth century. (In the early Massachusetts records the family name was spelled "Levett".) 

Leavitt attended Oberlin College before transferring to Harvard University's Society for the Collegiate Instruction of Women (later known as Radcliffe College), receiving a bachelor's degree in 1892. At Oberlin and Harvard, Leavitt studied a broad curriculum that included classical Greek, fine arts, philosophy, analytic geometry, and calculus. It wasn't until her fourth year of college that Leavitt took a course in astronomy, in which she earned an A–. Leavitt also began working as one of the women human "computers" at the Harvard College Observatory, hired by its director Edward Charles Pickering to measure and catalog the brightness of stars as they appeared in the observatory's photographic plate collection. (In the early 1900s, women were not allowed to operate telescopes.)

In 1893, Leavitt obtained credits toward a graduate degree in astronomy for her work at the Harvard College Observatory, but she never completed that degree. Leavitt left the observatory to make two trips to Europe and complete a stint as an art assistant at Beloit College in Wisconsin. At this time she contracted an illness that caused her increasingly to lose her hearing.

Astronomical career

Henrietta Swan Leavitt working at her desk in the Harvard College Observatory

 

Leavitt returned to the Harvard College Observatory in 1903. Because Leavitt had independent means, Pickering initially did not have to pay her. Later, she received $0.30 an hour for her work, being paid only $10.50 per week. She was reportedly "hard-working, serious-minded …, little given to frivolous pursuits and selflessly devoted to her family, her church, and her career". One of the women that Leavitt worked with in the Harvard Observatory was Annie Jump Cannon, who shared the experience of being deaf.

Pickering assigned Leavitt to the study of variable stars of the Small and Large Magellanic Clouds, as recorded on photographic plates taken with the Bruce Astrograph of the Boyden Station of the Harvard Observatory in Arequipa, Peru. She identified 1777 variable stars. In 1908 she published her results in the Annals of the Astronomical Observatory of Harvard College, noting that the brighter variables had the longer period.

In another paper published in 1912, Leavitt looked carefully at the relation between the periods and the brightness of a sample of 25 of the Cepheids variables in the Small Magellanic Cloud. This paper was communicated and signed by Edward Pickering, but the first sentence indicates that it was "prepared by Miss Leavitt". Leavitt made a graph of magnitude versus logarithm of period and determined that, in her own words, 

A straight line can be readily drawn among each of the two series of points corresponding to maxima and minima, thus showing that there is a simple relation between the brightness of the Cepheid variables and their periods.

Plot from a paper prepared by Leavitt in 1912. The horizontal axis is the logarithm of the period of the corresponding Cepheid, and the vertical axis is its magnitude. The lines drawn connect points corresponding to the stars' minimum and maximum brightness, respectively.

She then used the simplifying assumption that all of the Cepheids within the Small Magellanic Cloud were at approximately the same distance, so that their intrinsic brightness could be deduced from their apparent brightness as registered in the photographic plates, up to a scale factor since the distance to the Magellanic Clouds were as yet unknown. She expressed the hope that parallaxes to some Cepheids would be measured, which eventually happened thereby allowing her period-luminosity scale to be calibrated. This reasoning allowed Leavitt to establish that the logarithm of the period is linearly related to the logarithm of the star's average intrinsic optical luminosity (which is the amount of power radiated by the star in the visible spectrum).

Leavitt also developed, and continued to refine, the Harvard Standard for photographic measurements, a logarithmic scale that orders stars by brightness over 17 magnitudes. She initially analyzed 299 plates from 13 telescopes to construct her scale, which was accepted by the International Committee of Photographic Magnitudes in 1913.

Leavitt was a member of Phi Beta Kappa, the American Association of University Women, the American Astronomical and Astrophysical Society, the American Association for the Advancement of Science, and an honorary member of the American Association of Variable Star Observers. In 1921, when Harlow Shapley took over as director of the observatory, Leavitt was made head of stellar photometry. By the end of that year she had succumbed to cancer and was buried in the Leavitt family plot at Cambridge Cemetery in Cambridge, Massachusetts.

Illness and death

Leavitt family monument in Cambridge Cemetery

 

Leavitt's scientific work at Harvard was frequently interrupted by illness and family obligations. Her early death, at the age of 53, was seen as a tragedy by her colleagues for reasons that went beyond her scientific achievements. Her colleague Solon I. Bailey wrote in her obituary that "she had the happy faculty of appreciating all that was worthy and lovable in others, and was possessed of a nature so full of sunshine that, to her, all of life became beautiful and full of meaning."

"Sitting at the top of a gentle hill," writes George Johnson in his biography of Leavitt, "the spot is marked by a tall hexagonal monument, on top of which sits a globe cradled on a draped marble pedestal. Her uncle Erasmus Darwin Leavitt and his family also are buried there, along with other Leavitts. ..." A plaque memorializing Henrietta and her two siblings, Mira and Roswell, is mounted on one side of the monument. Nearby are the graves of Henry and William James. There is no epitaph at the grave site memorializing Henrietta Leavitt's achievements in astronomy.

Scientific impact

According to science writer Jeremy Bernstein, "variable stars had been of interest for years, but when she was studying those plates, I doubt Pickering thought she would make a significant discovery—one that would eventually change astronomy." The period–luminosity relationship for Cepheids, now known as "Leavitt's law" made the stars the first "standard candle" in astronomy, allowing scientists to compute the distances to galaxies too remote for stellar parallax observations to be useful. One year after Leavitt reported her results, Ejnar Hertzsprung determined the distance of several Cepheids in the Milky Way and that, with this calibration, the distance to any Cepheid could be accurately determined.

Cepheids were soon detected in other galaxies, such as Andromeda (notably by Edwin Hubble in 1923–24), and they became an important part of the evidence that "spiral nebulae" are independent galaxies located far outside of our own Milky Way. Thus, Leavitt's discovery would forever change our picture of the universe, as it prompted Harlow Shapley to move our Sun from the center of the galaxy in the "Great Debate" and Edwin Hubble to move our galaxy from the center of the universe.

Leavitt's discovery of a way to accurately measure distances on an inter-galactic scale, paved the way for modern astronomy's understanding of the structure and scale of the universe. The accomplishments of Edwin Hubble, the American astronomer who established that the universe is expanding, also were made possible by Leavitt's groundbreaking research. Hubble often said that Leavitt deserved the Nobel Prize for her work. Mathematician Gösta Mittag-Leffler, a member of the Swedish Academy of Sciences, tried to nominate her for that prize in 1924, only to learn that she had died of cancer three years earlier. (The Nobel Prize is not awarded posthumously.)

Posthumous honors

• The asteroid 5383 Leavitt and the crater Leavitt on the Moon are named after her to honor deaf men and women who have worked as astronomers.
• One of ASAS-SN telescopes, located in the McDonald Observatory in Texas, is named in her honor.

Books and plays

Lauren Gunderson wrote a play, Silent Sky, which followed Leavitt's journey from her acceptance at Harvard to her death.

George Johnson wrote a biography, Miss Leavitt's Stars, which showcases the triumphs of women's progress in science through the story of Leavitt.

Robert Burleigh wrote the biography Look Up!: Henrietta Leavitt, Pioneering Woman Astronomer for a younger audience. It is written for four to eight year olds.

Edwin Hubble (updated)

From Wikipedia, the free encyclopedia

Edwin Hubble
Born	Edwin Powell Hubble November 20, 1889 Marshfield, Missouri, U.S.
Died	September 28, 1953 (aged 63) San Marino, California, U.S.
Residence	United States
Nationality	American
Alma mater	University of Chicago The Queen's College, Oxford
Known for	Hubble sequence
Spouse(s)	Grace Burke Sr.
Awards	Newcomb Cleveland Prize 1924 Barnard Medal for Meritorious Service to Science 1935 Bruce Medal 1938 Franklin Medal 1939 Gold Medal of the Royal Astronomical Society 1940 Legion of Merit 1946
Scientific career
Fields	Astronomy
Institutions	University of Chicago Mount Wilson Observatory
Influenced	Allan Sandage
Signature

Edwin Powell Hubble (November 20, 1889 – September 28, 1953) was an American astronomer. He played a crucial role in establishing the fields of extragalactic astronomy and observational cosmology and is regarded as one of the most important astronomers of all time.

Hubble discovered that many objects previously thought to be clouds of dust and gas and classified as "nebulae" were actually galaxies beyond the Milky Way. He used the strong direct relationship between a classical Cepheid variable's luminosity and pulsation period (discovered in 1908 by Henrietta Swan Leavitt) for scaling galactic and extragalactic distances.

Hubble provided evidence that the recessional velocity of a galaxy increases with its distance from the earth, a property now known as "Hubble's law", despite the fact that it had been both proposed and demonstrated observationally two years earlier by Georges Lemaître. Hubble-Lemaître's Law implies that the universe is expanding. A decade before, the American astronomer Vesto Slipher had provided the first evidence that the light from many of these nebulae was strongly red-shifted, indicative of high recession velocities.

Hubble's name is most widely recognized for the Hubble Space Telescope which was named in his honor, with a model prominently displayed in his hometown of Marshfield, Missouri.

Biography

Edwin Hubble was born to Virginia Lee Hubble (née James) (1864–1934) and John Powell Hubble, an insurance executive, in Marshfield, Missouri, and moved to Wheaton, Illinois, in 1900. In his younger days, he was noted more for his athletic prowess than his intellectual abilities, although he did earn good grades in every subject except for spelling. Edwin was a gifted athlete, playing baseball, football, basketball, and running track in both high school and college. He played a variety of positions on the basketball court from center to shooting guard. In fact, Hubble even led the University of Chicago's basketball team to their first conference title in 1907. He won seven first places and a third place in a single high school track and field meet in 1906. 

His studies at the University of Chicago were concentrated on law, which led to a bachelor of science degree in 1910. Hubble also became a member of the Kappa Sigma Fraternity. He spent the three years at The Queen's College, Oxford after earning his bachelor's as one of the university's first Rhodes Scholars, initially studying jurisprudence instead of science (as a promise to his dying father), and later added literature and Spanish, and earning his master's degree.

In 1909, Hubble's father moved his family from Chicago to Shelbyville, Kentucky, so that the family could live in a small town, ultimately settling in nearby Louisville. His father died in the winter of 1913, while Edwin was still in England, and in the summer of 1913, Edwin returned to care for his mother, two sisters, and younger brother, as did his brother William. The family moved once more to Everett Avenue, in Louisville's Highlands neighborhood, to accommodate Edwin and William.

Hubble was also a dutiful son, who despite his intense interest in astronomy since boyhood, acquiesced to his father's request to study law, first at the University of Chicago and later at Oxford, though he managed to take a few math and science courses. After the death of his father in 1913, Edwin returned to the Midwest from Oxford but did not have the motivation to practice law. Instead, he proceeded to teach Spanish, physics and mathematics at New Albany High School in New Albany, Indiana, where he also coached the boys' basketball team. After a year of high-school teaching, he entered graduate school with the help of his former professor from the University of Chicago to study astronomy at the university's Yerkes Observatory, where he received his Ph.D. in 1917. His dissertation was titled "Photographic Investigations of Faint Nebulae". In Yerkes, he had access to one of the most powerful telescopes in the world at the time, which had an innovative 24 inch (61 cm) reflector.

Hubble's identity card in the American Expeditionary Forces.

 

After the United States declared war on Germany in 1917, Hubble rushed to complete his Ph.D. dissertation so he could join the military. Hubble volunteered for the United States Army and was assigned to the newly created 86th Division, where he served in 2nd Battalion, 343 Infantry Regiment. He rose to the rank of lieutenant colonel, and was found fit for overseas duty on July 9, 1918, but the 86th Division never saw combat. After the end of World War I, Hubble spent a year in Cambridge, where he renewed his studies of astronomy. In 1919, Hubble was offered a staff position at the Carnegie Institution for Science's Mount Wilson Observatory, near Pasadena, California, by George Ellery Hale, the founder and director of the observatory. Hubble remained on staff at Mount Wilson until his death in 1953. Shortly before his death, Hubble became the first astronomer to use the newly completed giant 200-inch (5.1 m) reflector Hale Telescope at the Palomar Observatory near San Diego, California. 

Hubble also worked as a civilian for U.S. Army at Aberdeen Proving Ground in Maryland during World War II as the Chief of the External Ballistics Branch of the Ballistics Research Laboratory during which he directed a large volume of research in exterior ballistics which increased the effective firepower of bombs and projectiles. His work was facilitated by his personal development of several items of equipment for the instrumentation used in exterior ballistics, the most outstanding development being the high-speed clock camera, which made possible the study of the characteristics of bombs and low-velocity projectiles in flight. The results of his studies were credited with greatly improving design, performance, and military effectiveness of bombs and rockets. For his work there, he received the Legion of Merit award.

Hubble was raised as a Christian but some of his later statements suggest uncertainty.

 

Hubble married Mrs. Grace Lillian (Burke) Leib (1889–1980), daughter of John Patrick and Luella (Kepford) Burke, on February 26, 1924. 

Hubble had a heart attack in July 1949 while on vacation in Colorado. He was taken care of by his wife and continued on a modified diet and work schedule. He died of cerebral thrombosis (a spontaneous blood clot in his brain) on September 28, 1953, in San Marino, California. No funeral was held for him, and his wife never revealed his burial site.

Discoveries

Universe goes beyond the Milky Way galaxy

The 100-inch Hooker telescope at Mount Wilson Observatory that Hubble used to measure galaxy distances and a value for the rate of expansion of the universe.

 

Edwin Hubble's arrival at Mount Wilson Observatory, California in 1919 coincided roughly with the completion of the 100-inch (2.5 m) Hooker Telescope, then the world's largest. At that time, the prevailing view of the cosmos was that the universe consisted entirely of the Milky Way Galaxy. Using the Hooker Telescope at Mt. Wilson, Hubble identified Cepheid variables (a kind of star that is used as a means to determine the distance from the galaxy) in several spiral nebulae, including the Andromeda Nebula and Triangulum. His observations, made in 1924, proved conclusively that these nebulae were much too distant to be part of the Milky Way and were, in fact, entire galaxies outside our own, suspected by researchers at least as early as 1755 when Immanuel Kant's General History of Nature and Theory of the Heavens appeared. This idea had been opposed by many in the astronomy establishment of the time, in particular by Harvard University-based Harlow Shapley. Despite the opposition, Hubble, then a thirty-five-year-old scientist, had his findings first published in The New York Times on November 23, 1924, then presented them to other astronomers at the January 1, 1925 meeting of the American Astronomical Society. Hubble's results for Andromeda were not formally published in a peer-reviewed scientific journal until 1929.

Hubble's classification scheme

 

Hubble's findings fundamentally changed the scientific view of the universe. Supporters state that Hubble's discovery of nebulae outside of our galaxy helped pave the way for future astronomers. Although some of his more renowned colleagues simply scoffed at his results, Hubble ended up publishing his findings on nebulae. This published work earned him an award titled the American Association Prize and five hundred dollars from Burton E. Livingston of the Committee on Awards.

Hubble also devised the most commonly used system for classifying galaxies, grouping them according to their appearance in photographic images. He arranged the different groups of galaxies in what became known as the Hubble sequence.

Redshift increases with distance

Hubble went on to estimate the distances to 24 extra-galactic nebulae, using a variety of methods. In 1929 Hubble examined the relation between these distances and their radial velocities as determined from their redshifts. His estimated distances are now known to all be too small, by up to a factor of about 7. This was due to factors such as the fact that there are two kinds of Cepheid variables or confusing bright gas clouds with bright stars. However, his distances were more or less proportional to the true distances, and combining his distances with measurements of the redshifts of the galaxies by Vesto Slipher, and by his assistant Milton L. Humason, he found a roughly linear relation between the distances of the galaxies and their radial velocities (corrected for solar motion), a discovery that later became known as Hubble's law. 

This meant, the greater the distance between any two galaxies, the greater their relative speed of separation. If interpreted that way, Hubble's measurements on 46 galaxies lead to a value for the Hubble Constant of 500 km/s/Mpc, which is much higher than the currently accepted value of 70 km/s/Mpc due to errors in their distance calibrations.

Yet the reason for the redshift remained unclear. Georges Lemaître, a Belgian Catholic priest and physicist, predicted on theoretical grounds based on Einstein's equations for general relativity the redshift-distance relation, and published observational support for it, two years before the discovery of Hubble's law. However, many cosmologists and astronomers (including Hubble himself) failed to recognize the work of Lemaître; Hubble remained doubtful about Lemaître's interpretation for his entire life. Although he used the term "velocities" in his paper (and "apparent radial velocities" in the introduction), he later expressed doubt about interpreting these as real velocities. In 1931 he wrote a letter to the Dutch cosmologist Willem de Sitter expressing his opinion on the theoretical interpretation of the redshift-distance relation:

Mr. Humason and I are both deeply sensible of your gracious appreciation of the papers on velocities and distances of nebulae. We use the term 'apparent' velocities to emphasize the empirical features of the correlation. The interpretation, we feel, should be left to you and the very few others who are competent to discuss the matter with authority.

Today, the "apparent velocities" in question are usually thought of as an increase in proper distance that occurs due to the expansion of the universe. Light traveling through an expanding metric will experience a Hubble-type redshift, a mechanism somewhat different from the Doppler effect (although the two mechanisms become equivalent descriptions related by a coordinate transformation for nearby galaxies). 

In the 1930s, Hubble was involved in determining the distribution of galaxies and spatial curvature. These data seemed to indicate that the universe was flat and homogeneous, but there was a deviation from flatness at large redshifts. According to Allan Sandage,

Hubble believed that his count data gave a more reasonable result concerning spatial curvature if the redshift correction was made assuming no recession. To the very end of his writings he maintained this position, favouring (or at the very least keeping open) the model where no true expansion exists, and therefore that the redshift "represents a hitherto unrecognized principle of nature.

There were methodological problems with Hubble's survey technique that showed a deviation from flatness at large redshifts. In particular, the technique did not account for changes in luminosity of galaxies due to galaxy evolution. Earlier, in 1917, Albert Einstein had found that his newly developed theory of general relativity indicated that the universe must be either expanding or contracting. Unable to believe what his own equations were telling him, Einstein introduced a cosmological constant (a "fudge factor") to the equations to avoid this "problem". When Einstein learned of Hubble's redshifts, he immediately realized that the expansion predicted by general relativity must be real, and in later life he said that changing his equations was "the biggest blunder of [his] life." In fact, Einstein apparently once visited Hubble and tried to convince him that the universe was expanding.

Hubble also discovered the asteroid 1373 Cincinnati on August 30, 1935. In 1936 he wrote The Observational Approach to Cosmology and The Realm of the Nebulae which explained his approaches to extra-galactic astronomy and his view of the subject's history.

In December 1941, Hubble reported to the American Association for the Advancement of Science that results from a six-year survey with the Mt. Wilson telescope did not support the expanding universe theory. According to an LA Times article reporting on Hubble's remarks, "The nebulae could not be uniformly distributed, as the telescope shows they are, and still fit the explosion idea. Explanations which try to get around what the great telescope sees, he said, fail to stand up. The explosion for example would have had to start long after the earth was created, and possibly even after the first life appeared here." (Hubble's estimate of what we now call the Hubble constant would put the Big Bang only 2000 million years ago.)

Accusations concerning Lemaître's priority

In 2011 the journal Nature reported claims that Hubble had played a role in the redaction of key parts of the translation of Lemaître's 1927 paper, which stated what is now called Hubble's Law and also gave observational evidence for it. Historians quoted in the article were skeptical that the redactions were part of a campaign to ensure Hubble retained priority. However, the observational astronomer Sidney van den Bergh published a paper suggesting that while the omissions may have been made by a translator, they may still have been deliberate.

In November 2011, the astronomer Mario Livio reported in Nature that documents in the Lemaître archive demonstrated that the redaction had indeed been carried out by Lemaître himself, who apparently saw little point in including scientific content which had already been reported by Hubble.

No Nobel Prize

At the time, the Nobel Prize in Physics did not recognize work done in astronomy. Hubble spent much of the later part of his career attempting to have astronomy considered an area of physics, instead of being its own science. He did this largely so that astronomers—including himself—could be recognized by the Nobel Prize Committee for their valuable contributions to astrophysics. This campaign was unsuccessful in Hubble's lifetime, but shortly after his death, the Nobel Prize Committee decided that astronomical work would be eligible for the physics prize. However, the prize is not one that can be awarded posthumously.

Stamp

On March 6, 2008, the United States Postal Service released a 41-cent stamp honoring Hubble on a sheet titled "American Scientists" designed by artist Victor Stabin. His citation reads:

Often called a "pioneer of the distant stars," astronomer Edwin Hubble (1889–1953) played a pivotal role in deciphering the vast and complex nature of the universe. His meticulous studies of spiral nebulae proved the existence of galaxies other than our own Milky Way. Had he not died suddenly in 1953, Hubble would have won that year's Nobel Prize in Physics.

(Note that the assertion that he would have won the Nobel Prize in 1953 is likely false, although he was nominated for the prize that year.)

The other scientists on the "American Scientists" sheet include Gerty Cori, biochemist; Linus Pauling, chemist, and John Bardeen, physicist.

Honors

Awards

• Newcomb Cleveland Prize in 1924;
• Bruce Medal in 1938;
• Franklin Medal in 1939;
• Gold Medal of the Royal Astronomical Society in 1940;
• Legion of Merit for outstanding contribution to ballistics research in 1946;

Namesakes

• Asteroid 2069 Hubble;
• The crater Hubble on the Moon;
• Orbiting Hubble Space Telescope;
• Edwin P. Hubble Planetarium, located in the Edward R. Murrow High School, Brooklyn, NY.;
• Edwin Hubble Highway, the stretch of Interstate 44 passing through his birthplace of Marshfield, Missouri;
• Hubble Middle School, a public school in Wheaton, Illinois where he lived from 11 years old and up.

Other notable appearances

• Hall of Famous Missourians 2003
• 2008 "American Scientists" US stamp series, $0.41.
• 2017 Indiana Basketball Hall of Fame

In popular culture

The play Creation's Birthday, written by Cornell physicist Hasan Padamsee, tells Hubble's life story.

A famous quote by Edwin Hubble goes: "Equipped with his five senses man explores the universe around him and calls the adventure Science".

In the popular documentary Cosmos: A Personal Voyage by astronomer Carl Sagan, Hubble's life and work are portrayed on screen in episode 10: The Edge of Forever.

 

His work on Red Shift was immortalized in a limerick by Alexander Rolfe:

Thanks to Edwin P. Hubble

Our static cosmology was in serious trouble--

When we saw a wavelength

Of such tiny strength,

It proved the universe was an expanding bubble.

Messier 87

From Wikipedia, the free encyclopedia

Messier 87

Messier 87 as seen by the Hubble Space Telescope
Observation data Epoch J2000
Constellation	Virgo
Right ascension	12h 30m 49.42338s
Declination	+12° 23′ 28.0439″
Apparent dimension (V)	7.2 × 6.8 moa
Apparent magnitude (V)	9.59
Characteristics
Type	E+0-1 pec, NLRG Sy
Astrometry
Heliocentric radial velocity	1307 ± 7 km/s
Redshift	0.004360 ± 0.000022
Galactocentric velocity	1254 ± 7 km/s
Distance	53.5 ± 1.63 Mly (16.40 ± 0.50 Mpc)
Other designations
Virgo A, Virgo X-1, NGC 4486, UGC 7654, PGC 41361, VCC 1316, Arp 152, 3C 274, 3U 1228+12.
Database references
SIMBAD	data

Messier 87 (also known as Virgo A or NGC 4486, generally abbreviated to M87) is a supergiant elliptical galaxy in the constellation Virgo. One of the most massive galaxies in the local Universe, it is notable for its large population of globular clusters—about 12,000 compared to the 150–200 orbiting the Milky Way—and its jet of energetic plasma that originates at the core and extends at least 1,500 parsecs (4,900 light-years), traveling at relativistic speed. It is one of the brightest radio sources in the sky, and a popular target for both amateur and professional astronomers.

The French astronomer Charles Messier discovered M87 in 1781, and catalogued it as a nebulous feature while searching for objects that would otherwise confuse comet hunters. M87 is located about 16.4 million parsecs (53 million light-years) from Earth and is the second-brightest galaxy within the northern Virgo Cluster, having many satellite galaxies. Unlike a disk-shaped spiral galaxy, M87 has no distinctive dust lanes. Instead, it has an almost featureless, ellipsoidal shape typical of most giant elliptical galaxies, diminishing in luminosity with distance from the center. Forming around one sixth of its mass, M87's stars have a nearly spherically symmetric distribution. Their population density decreases with increasing distance from the core. It has an active supermassive black hole at its core, which forms the primary component of an active galactic nucleus.

The galaxy is a strong source of multiwavelength radiation, particularly radio waves. Its galactic envelope extends to a radius of about 150 kiloparsecs (490 thousand light-years), where it is truncated—possibly by an encounter with another galaxy. Its interstellar medium consists of diffuse gas enriched by elements emitted from evolved stars.

Observation history

In 1781, the French astronomer Charles Messier published a catalogue of 103 objects that had a nebulous appearance as part of a list intended to identify objects that might otherwise be confused with comets. In subsequent use, each catalogue entry was prefixed with an "M". Thus, M87 was the eighty-seventh object listed in Messier's catalogue. During the 1880s, the object was included as NGC 4486, in the New General Catalogue of nebulae and star clusters assembled by the Danish-Irish astronomer John Dreyer, which he based primarily on the observations of the English astronomer John Herschel.

In 1918, the American astronomer Heber Curtis of Lick Observatory noted M87's lack of a spiral structure and observed a "curious straight ray ... apparently connected with the nucleus by a thin line of matter." The ray appeared brightest near the galactic center. The following year, a supernova within M87 reached a peak photographic magnitude of 21.5, although this event was not reported until photographic plates were examined by the Russian astronomer Innokentii A. Balanowski in 1922.

Identification as galaxy

In Hubble's galaxy classification scheme, M87 is an E0 galaxy

 

In 1922, the American astronomer Edwin Hubble categorized M87 as one of the brighter globular nebulae, as it lacked any spiral structure, but like spiral nebulae, appeared to belong to the family of non-galactic nebulae. In 1926 he produced a new categorization, distinguishing extragalactic from galactic nebulae, the former being independent star systems. M87 was classified as a type of elliptical extragalactic nebula with no apparent elongation (class E0).

In 1931, Hubble described M87 as a member of the Virgo Cluster, and gave a provisional estimate of 1.8 million parsecs (5.9 million light-years) from Earth. It was then the only known elliptical nebula for which individual stars could be resolved, although it was pointed out that globular clusters would be indistinguishable from individual stars at such distances. In his 1936 The Realm of the Nebulae, Hubble examines the terminology of the day; some astronomers labeled extragalactic nebulae as external galaxies on the basis that they were stellar systems at far distances from our own galaxy, while others preferred the conventional term extragalactic nebulae, as galaxy then was synonym for the Milky Way. M87 continued to be labelled as an extragalactic nebula at least until 1954.

Spiral flow of the black-hole-powered jet, imaged with the Hubble Space Telescope

Modern research

In 1947, a prominent radio source, Virgo A, was identified overlapping the location of M87. The source was confirmed to be M87 by 1953, and the linear relativistic jet emerging from the core of the galaxy was suggested as the cause. This jet extended from the core at a position angle of 260° to an angular distance of 20″ with an angular width of 2″. In 1969–70, a strong component of the radio emission was found to closely align with the optical source of the jet.

In 1966, the United States Naval Research Laboratory's Aerobee 150 rocket identified Virgo X-1, the first X-ray source in Virgo. The Aerobee rocket launched from White Sands Missile Range on 7 July 1967, yielded further evidence that the source of Virgo X-1 was the radio galaxy M87. Subsequent X-ray observations by the HEAO 1 and Einstein Observatory showed a complex source that included the active galactic nucleus of M87. However, there is little central concentration of the X-ray emission.

M87 has been an important testing ground for techniques that measure the masses of central supermassive black holes in galaxies. In 1978, stellar-dynamical modeling of the mass distribution in M87 gave evidence for a central mass of five billion solar masses. After the installation of the COSTAR corrective-optics module in the Hubble Space Telescope in 1993, the Hubble Faint Object Spectrograph (FOS) was used to measure the rotation velocity of the ionized gas disk at the center of M87, as an "early release observation" designed to test the scientific performance of the post-repair Hubble instruments. The FOS data indicated a central black hole mass of 2.4 billion solar masses, with 30% uncertainty.

Visibility

Area in constellation Virgo around M87

 

M87 is located near the high declination border of the Virgo constellation, next to the constellation of Coma Berenices. It lies along the line between the stars Epsilon Virginis and Denebola. At an apparent magnitude of 9.59, the galaxy can be observed using a small telescope with a 6 cm (2.4 in) aperture, extending across an angular area of 7.2 × 6.8 arcminutes with a bright, 45-arcsecond core. Viewing the jet is a challenge without the aid of photography. Before 1991, the Russian-American astronomer Otto Struve was the only person known to have seen the jet visually, using the 254 cm (100 in) Hooker telescope. In more recent years it has been observed in larger amateur telescopes under excellent conditions.

Properties

Huge halo around Messier 87

 

In the modified Hubble sequence galaxy morphological classification scheme of the French astronomer Gérard de Vaucouleurs, M87 is categorized as an E0p galaxy. "E0" designates an elliptical galaxy that displays no flattening—that is, it appears spherical. A "p" suffix indicates a peculiar galaxy that does not fit cleanly into the classification scheme; in this case, the peculiarity is the presence of the jet emerging from the core. In the Yerkes (Morgan) scheme, M87 is classified as a type-cD galaxy. A D galaxy has an elliptical-like nucleus surrounded by an extensive, dustless, diffuse envelope. A D type supergiant is called a cD galaxy.

The distance to M87 has been estimated using several independent techniques. These include measurement of the luminosity of planetary nebulae, comparison with nearby galaxies whose distance is estimated using standard candles such as cepheid variables, the linear size distribution of globular clusters, and the tip of the red-giant branch method using individually resolved red giant stars. These measurements are consistent with each other, and their weighted average yields a distance estimate of 16.4 ± 0.5 megaparsecs (53.5 ± 1.63 million light-years).

M87 is one of the most massive galaxies in the local Universe. It spans a diameter of 120 thousand light-years, which is slightly lower than that of the Milky Way, but M87 is a spheroid, not a flat spiral. The mass of M87 within a radius (r) of 9–40 kiloparsecs (29–130 thousand light-years) from the core steadily increases roughly in proportion to r^1.7. Within a radius of 32 kiloparsecs (100 thousand light-years), the mass is (2.4 ± 0.6) × 10¹² times the mass of the Sun, which is double the mass of the Milky Way galaxy. Only a fraction of this mass is in the form of stars. M87 has an estimated mass to luminosity ratio of 6.3 ± 0.8; that is, about one part in six of the galaxy's mass is in the form of stars that radiate energy. The total mass of M87 may be 200 times that of the Milky Way.

Stellar velocity map of the central region of M87, showing the motion of stars relative to Earth. Blue patches represent motion towards the Earth and red ones away from the Earth, while yellow and green are in between the two extremes. The map signifies random motion of the stars.

 

Gas is infalling into the galaxy at the rate of two to three solar masses per year, most of which may be accreted onto the core region. The extended stellar envelope of this galaxy reaches a radius of about 150 kiloparsecs (490 thousand light-years), compared to about 100 kiloparsecs (330 thousand light-years) for the Milky Way. Beyond that distance the outer edge of the galaxy has been truncated by some means; possibly by an earlier encounter with another galaxy. There is evidence of linear streams of stars to the northwest of the galaxy, which may have been created by tidal stripping of orbiting galaxies or by small satellite galaxies falling in toward M87. Moreover, a filament of hot, ionized gas in the northeastern outer part of the galaxy may be the remnant of a small, gas-rich galaxy that was disrupted by M87 and could be feeding its active nucleus. M87 is estimated to have at least 50 satellite galaxies, including NGC 4486B and NGC 4478.

The spectrum of the nuclear region of M87 shows the emission lines of various ions, including hydrogen (HI, HII), helium (HeI), oxygen (OI, OII, OIII), nitrogen (NI), magnesium (MgII) and sulfur (SII). The line intensities for weakly ionized atoms (such as neutral atomic oxygen, OI) are stronger than those of strongly ionized atoms (such as doubly ionized oxygen, OIII). A galactic nucleus with such spectral properties is termed a LINER, for "low-ionization nuclear emission-line region". The mechanism and source of weak-line-dominated ionization in LINERs and M87 are under debate. Possible causes include shock-induced excitation in the outer parts of the disk or photoionization in the inner region powered by the jet.

Elliptical galaxies such as M87 are believed to form as the result of one or more mergers of smaller galaxies. They generally contain relatively little cold interstellar gas (in comparison with spiral galaxies) and they are populated mostly by old stars, with little or no ongoing star formation. M87's elliptical shape is maintained by the random orbital motions of its constituent stars, in contrast to the more orderly rotational motions found in a spiral galaxy such as the Milky Way. Using the Very Large Telescope to study the motions of about 300 planetary nebulae, astronomers have determined that M87 absorbed a medium-sized star-forming spiral galaxy over the last billion years. This has resulted in the addition of some younger, bluer stars to M87. The distinctive spectral properties of the planetary nebulae allowed astronomers to discover a chevron-like structure in M87's halo which was produced by the incomplete phase-space mixing of a disrupted galaxy.

Components

Supermassive black hole

Artist's concept of a supermassive black hole and its accretion disk

 

The core contains a supermassive black hole that weighs billions of times the Sun's mass: estimates have ranged from (3.5 ± 0.8) × 10⁹ M_☉ to (6.6 ± 0.4) × 10⁹ M_☉, with the latest measurement being 7.22+0.34
−0.40×10⁹ M_☉. This mass is one of the highest known for such an object. A rotating disk of ionized gas surrounds the black hole, and is roughly perpendicular to the relativistic jet. The disk rotates at velocities of up to roughly 1,000 km/s, and spans a maximum diameter of 0.12 parsecs (0.39 light-years). Gas accretes onto the black hole at an estimated rate of one solar mass every ten years (about 90 earth masses per day).

Observation suggests that the black hole may be displaced from the galactic center by about seven parsecs (23 light-years). The displacement is in the opposite direction of the one-sided jet, which may indicate that the black hole was accelerated away by the jet. Another possibility is that the change in location occurred during the merger of two supermassive black holes. A 2011 study did not find any statistically significant displacement.

Interstellar medium

The space between the stars in M87 is filled with a diffuse interstellar medium of gas that has been chemically enriched by the elements ejected from stars as they passed beyond their main sequence lifetime. Carbon and nitrogen are continuously supplied by stars of intermediate mass as they pass through the asymptotic giant branch. The heavier elements from oxygen to iron are produced largely by supernova explosions within the galaxy. Of the heavy elements, about 60% were produced by core-collapse supernovae, while the remainder came from type Ia supernovae. The distribution of oxygen is roughly uniform throughout, at about half of the solar value (i.e., oxygen abundance in the Sun), while iron distribution peaks near the center where it approaches the solar iron value. Since oxygen is produced mainly by core-collapse supernovae, which occur during the early stages of galaxies and mostly in outer star-forming regions, the distribution of these elements suggests an early enrichment of the interstellar medium from core-collapse supernovae and a continuous contribution from Type Ia supernovae throughout the history of M87. The contribution of elements from these sources was much lower than in the Milky Way.

Examination of M87 at far infrared wavelengths shows an excess emission at wavelengths longer than 25 μm. Normally, this may be an indication of thermal emission by warm dust. In the case of M87, the emission can be fully explained by synchrotron radiation from the jet; within the galaxy, silicate grains are expected to survive for no more than 46 million years because of the X-ray emission from the core. This dust may be destroyed by the hostile environment or expelled from the galaxy. The combined mass of dust in M87 is no more than 70,000 times the mass of the Sun. By comparison, the Milky Way's dust equals about a hundred million (10⁸) solar masses.

Although M87 is an elliptical galaxy and therefore lacks the dust lanes of a spiral galaxy, optical filaments have been observed in it, which arise from gas falling towards the core. Emission probably comes from shock-induced excitation as the falling gas streams encounter X-rays from the core region. These filaments have an estimated mass of about 10,000 solar masses. Surrounding the galaxy is an extended corona with hot, low-density gas.

Globular clusters

M87 has an abnormally large population of globular clusters. A 2006 survey out to an angular distance of 25′ from the core estimates that there are 12,000 ± 800 globular clusters in orbit around M87, compared to 150–200 in and around the Milky Way. The clusters are similar in size distribution to those of the Milky Way, most having an effective radius of 1 to 6 parsecs. The size of the M87 clusters gradually increases with distance from the galactic center. Within a four-kiloparsec (13,000-light-year) radius of the core, the cluster metallicity—the abundance of elements other than hydrogen and helium—is about half the abundance in the Sun. Outside this radius, metallicity steadily declines as the cluster distance from the core increases. Clusters with low metallicity are somewhat larger than metal-rich clusters. In 2014, HVGC-1, the first hypervelocity globular cluster, was discovered escaping from M87 at 2,300 km/s. The escape of the cluster with such a high velocity may have been the result of a close encounter with, and subsequent gravitational kick from, a supermassive black hole binary. If this interpretation is correct, the core of M87 would have two supermassive black holes, the result of an ancient collision between two galaxies which merged into a single giant galaxy.

Almost a hundred ultra-compact dwarfs have been identified in M87. They resemble globular clusters but have a diameter of ten parsecs (33 light-years) or more, much larger than the three-parsec (9.8-light-year) maximum of globular clusters. It is unclear whether they are dwarf galaxies captured by M87 or a new class of massive globular cluster.

Jet

This Hubble Space Telescope photograph shows the jet of matter ejected from M87 at nearly the speed of light, as it stretches 1.5 kpc (5 kly) from the galactic core

 

In this X-ray (Chandra) and radio (VLA) composite image, hot matter (blue in X-ray) from the Virgo cluster falls toward the core of M87 and cools, where it is met by the relativistic jet (orange in radio), producing shock waves in the galaxy's interstellar medium

 

The relativistic jet of matter emerging from the core extends at least 1.5 kiloparsecs (5,000 light-years) from the nucleus and consists of matter ejected from a supermassive black hole. The jet is highly collimated, appearing constrained to an angle of 60° within 0.8 parsecs (2.6 light-years) of the core, to about 16° at two parsecs (6.5 light-years), and to 6–7° at twelve parsecs (39 light-years). Its base has the diameter of 5.5 ± 0.4 Schwarzschild radii, and is probably powered by a prograde accretion disk around a spinning supermassive black hole. The German-American astronomer Walter Baade found that light from the jet was plane polarized, which suggests that the energy is generated by the acceleration of electrons moving at relativistic velocities in a magnetic field. The total energy of these electrons is estimated at 5.1 × 10⁵⁶ ergs (5.1 × 10⁴⁹ joules or 3.2 × 10⁶⁸ eV). This is roughly 10¹³ times the energy produced by the Milky Way in one second, which is estimated at 5 × 10³⁶ joules. The jet is surrounded by a lower-velocity non-relativistic component. There is evidence of a counter jet, but it remains unseen from the Earth due to relativistic beaming. The jet is precessing, causing the outflow to form a helical pattern out to 1.6 parsecs (5.2 light-years). Lobes of expelled matter extend out to 80 kiloparsecs (260 thousand light-years).

In pictures taken by the Hubble Space Telescope in 1999, the motion of M87's jet was measured at four to six times the speed of light. This phenomenon, called superluminal motion, is an illusion caused by the relativistic velocity of the jet. The time interval between any two light pulses emitted by the jet is, as registered by the observer, less than the actual interval due to the relativistic speed of the jet moving in the direction of the observer. This results in perceived faster-than-light speeds. Detection of such motion is used to support the theory that quasars, BL Lacertae objects and radio galaxies may all be the same phenomenon, known as active galaxies, viewed from different perspectives. It is proposed that M87 is a BL Lacertae object (with a low-luminosity nucleus compared with the brightness of its host galaxy) seen from a relatively large angle. Flux variations, characteristic of the BL Lacertae objects, have been observed in M87.

Radio wavelength image of M87 showing strong radio emission from the core

 

Observations indicate that the rate at which material is ejected from the supermassive black hole is variable. These variations produce pressure waves in the hot gas surrounding M87. The Chandra X-ray Observatory has detected loops and rings in the gas. Their distribution suggests that minor eruptions occur every few million years. One of the rings, caused by a major eruption, is a shock wave 26 kiloparsecs (85 thousand light-years) in diameter around the black hole. Other features observed include narrow X-ray-emitting filaments up to 31 kiloparsecs (100 thousand light-years) long, and a large cavity in the hot gas caused by a major eruption 70 million years ago. The regular eruptions prevent a huge reservoir of gas from cooling and forming stars, implying that M87's evolution may have been seriously affected, preventing it from becoming a large spiral galaxy. These observations also indicate that the variable eruptions produce sound waves of about 56 to 59 octaves below middle C in the medium.

M87 is a very strong source of gamma rays, the most energetic rays of the electromagnetic spectrum. Gamma rays emitted by M87 have been observed since the late 1990s. In 2006, using the High Energy Stereoscopic System Cherenkov telescopes, scientists measured the variations of the gamma ray flux coming from M87, and found that the flux changes over a matter of days. This short period indicates that the most likely source of the gamma rays is a supermassive black hole. In general, the smaller the diameter of the emission source, the faster the variation in flux, and vice versa.

A knot of matter in the jet (designated HST-1), about 65 parsecs (210 light-years) from the core, has been tracked by the Hubble Space Telescope and the Chandra X-ray Observatory. By 2006, the X-ray intensity of this knot had increased by a factor of 50 over a four-year period, while the X-ray emission has since been decaying in a variable manner.

The interaction of relativistic jets of plasma emanating from the core with the surrounding medium gives rise to radio lobes in active galaxies. The lobes occur in pairs and are often symmetrical. The two radio lobes of M87 together span about 80 kiloparsecs; the inner parts, extending up to two kiloparsecs, emit strongly at radio wavelengths. Two flows of material emerge from this region, one aligned with the jet itself and the other in the opposite direction. The flows are asymmetrical and deformed, implying that they encounter a dense intracluster medium. At greater distances, both flows diffuse into two lobes. The lobes are surrounded by a fainter halo of radio-emitting gas.

Environment

Messier 87 is a member of the Virgo Cluster, and can be seen to lower left of this cluster image.

 

M87 is near the center of the Virgo Cluster, a closely compacted structure of about 2,000 members. It forms the core of the larger Virgo Supercluster, of which the Local Group (including the Milky Way) is an outlying member. It is organized into at least three distinct subsystems associated with the three large galaxies—M87, M49 and M86—with the subgroup centered around M87 (Virgo A) and M49 (Virgo B). There is a preponderance of elliptical and S0 galaxies around M87, with a chain of elliptical galaxies aligned with the jet. In terms of mass, M87 is a dominant member of the cluster, and hence appears to be moving very little relative to the cluster as a whole. It is defined as the cluster center. The cluster has a sparse gaseous atmosphere that emits X-rays that decrease in temperature toward the middle, where M87 is located. The combined mass of the cluster is estimated to be (0.15–1.5) × 10¹⁵ solar masses.

Measurements of the motion of intracluster planetary nebulae between M87 and M86 suggest that the two galaxies are moving toward each other and that this may be their first encounter. M87 may have interacted with M84 in the past, as evidenced by the truncation of M87's outer halo by tidal interactions. The truncated halo may also have been caused by contraction due to an unseen mass falling into M87 from the rest of the cluster, which may be the hypothesized dark matter. A third possibility is that the halo's formation was truncated by early feedback from the active galactic nucleus at the core of M87.