Sandage, Allan (1926 - 2010 Nov 13)

[bystander]

Tribute to a Pioneering Cosmologist
Sky & Telescope | 15 Nov 2010

When Allan Sandage died on November 13th, succumbing to pancreatic cancer at age 84, we lost one of the true giants of observational astronomy.

[Ann]

Allan Sandage, 1926-2010
KEITH COOPER
ASTRONOMY NOW
Posted: 16 November 2010

http://www.astronomynow.com/news/n1011/16Sandage/

Allan Sandage, one of the greatest astronomers of the twentieth century, has died at the age of 84. Sandage, who worked with Edwin Hubble as an assistant at Mount Wilson Observatory, played an integral part in increasing our understanding of the scale of the Universe and determining the Hubble Constant, which describes the Universe’s expansion.

In the 1920s and 1930s, Edwin Hubble had become famous for discovering that the spiral nebulae were in fact galaxies beyond ours, and that the Universe was expanding. As Hubble’s protegé, Sandage never came close to achieving his mentor’s celebrity status, nor did he try to, but while Hubble set the ball rolling, it was Sandage who corrected his mistakes and introduced unprecedented accuracy into the measurements of the Universe, continuing the tradition of showing how what we think we know is merely the tip of the iceberg.

He graduated from the University of Illinois in 1948, and achieved a PhD from Caltech in 1953 under the tutelage of Walter Baade, while at the same time working as an assistant to Hubble at Mount Wilson. In 1950 Baade had discovered that Hubble’s estimate of the distance to the Andromeda Galaxy, 700,000 light years, was half too small, based on the identification of Cepheid variables in Andromeda. After Hubble’s passing in 1953, Sandage continued his mentor’s research.

In 1958 Sandage won the American Astronomical Society’s Warner Prize for outstanding achievement by a young astronomer, based on his work on the physics and evolution of stars, allowing him to develop a method of calculating the ages of stars that have left the main sequence of the Hertzsprung–Russell diagram. At the awards ceremony, in front of the great and the good of American astronomy, he presented revelatory new results: using the Hale Telescope, he had re-worked Hubble’s distance measurements to the galaxies having found several miscalculations in them (not of any fault of Hubble’s – he was limited by the telescopes of the time). Hubble had misidentified various objects in several distant galaxies, such as globular clusters and bright knots of star-forming gas, as supergiant stars with which he had estimated distances based on their brightness. Hubble had known this might prove to be a problem – from a distance a globular cluster of hundreds of thousands of stars can look like one massive star. Consequently Hubble’s distance ladder was scaled incorrectly, explained Sandage. Sandage showed that the Andromeda Galaxy was two million light years away. Hubble had estimated that the Virgo Cluster was seven million light years distant; Sandage proved that it was more like 50 million light years.

And so on. Literally overnight Sandage had expanded the Universe to unimaginable scales, just as Hubble has done before him, and this resulted in a more accurate assessment of Hubble’s Constant, which is the measure of the expansion of the Universe, of 75 kilometres per second per megaparsec (today’s best measurements give the Hubble Constant as 71 kilometres per second per megaparsec, showing how close Sandage was). This allowed him to estimate the age of the Universe as 15 billion years, not too far away from today’s established age of 13.7 billion years.

Sandage, working at the Carnegie Observatories in California, continued his efforts to hone the Hubble Constant, in his latter years using the Hubble Space Telescope to observe extragalactic Type Ia supernovae as standard candles. He also discovered the starburst activity and black hole jets in the galaxy M82, and published the comprehensive Hubble Atlas of Galaxies, in 1961. He continued to research and publish papers right up until his death, his last paper on RR Lyrae variable stars (another kind of standard candle) appearing in the Astrophysical Journal in June. A true giant of astronomy, he published more than 500 scientific papers in total throughout his career.

Me being me, I'll best remember Sandage for publishing a paper where he said that unreddened supernovae type Ia are blue, with a color index of -0.012 (I think). (What he really said was that unreddened supernovae type Ia, which has a color index of -0.012, has a peak absolute magnitude of -19.3. Reddened supernovae type Ia will appear to be fainter.)

Since then I always note the color of supernovae in color pictures of galaxies, and if a supernova appears blue, it has so far always turned out to be a type Ia.

Click to view full size image

Supernova 2007ck on the left is yellowish and not so bright, so we may suspect that it is a type II supernova. And indeed it is. Supernova 2007co on the right is brighter and noticeably blue, so we may strongly suspect that it is a type Ia event. And indeed it is.

Thanks for teaching me this, Alan Sandage.

Ann

[neufer]

Me being me, I'll best remember Sandage for publishing a paper where he said that unreddened supernovae type Ia are blue, with a color index of -0.012 (I think). (What he really said was that unreddened supernovae type Ia, which has a color index of -0.012, has a peak absolute magnitude of -19.3. Reddened supernovae type Ia will appear to be fainter.)

Since then I always note the color of supernovae in color pictures of galaxies, and if a supernova appears blue, it has so far always turned out to be a type Ia.

Supernova 2007ck is yellowish and not so bright, so we may suspect that it is a type II supernova. And indeed it is.
Supernova 2007co is brighter and noticeably blue, so we may strongly suspect that it is a type Ia event. And indeed it is.

Thanks for teaching me this, Alan Sandage.

Type II supernova are blue with a strong red hydrogen component; hence they are purple.
(Type Ib are yellow & Type Ic are green.)

Brightness with time

Spectra

Light curve of SN 2006gy (uppermost intermittent squares) compared with other types of supernovae.

This diagram illustrates the pair production process that astronomers think triggered the explosion in SN 2006gy. A sufficiently massive star can produce gamma rays of such high energy that some of the photons convert into pairs of electrons and positrons causing a runaway reaction which destroys the star.

<<SN 2006gy was an extremely energetic supernova, sometimes referred to as a hypernova or quark-nova, that was discovered on September 18, 2006. On May 7, 2007, NASA and several of the astronomers announced the first detailed analyses of the supernova, describing it as the "brightest stellar explosion ever recorded". In October 2007 Quimby announced that SN 2005ap had broken SN 2006gy's record as the brightest ever recorded supernova.

SN 2006gy occurred in a distant galaxy (NGC 1260), approximately 238 million light years (72 megaparsecs) away. Therefore, due to the time it took light from the supernova to reach Earth, the event occurred about 238 million years ago. Preliminary indications are that it was an unusually high-energy supernova of a very large star, around 150 solar masses (M⊙), possibly of a type referred to as a pair-instability supernova.

A pair instability supernova can only happen in stars that are very massive—having a range of around 130 to 250 solar masses. The massive star's core can produce high energy gamma rays which have a greater energy than the rest mass of two electrons (mass-energy equivalence). These gamma rays interact with electromagnetic fields of the atomic nuclei in the star, and become particle and anti-particle pairs of electrons and positrons. This causes the average travel distance of the gamma rays to become shorter, causing the temperature of the interior of the star to rise. This causes an even larger fraction of the produced gamma rays to be of high enough energy for pair production, causing more of the energy to be reabsorbed closer to its source. This creates a runaway reaction. As the energy is concentrated more and more into the star's core, the outer layers start to fall inwards, which then compress the core. The compression and heating produce a rapid (few seconds) thermonuclear burn or explosion of the core material. The explosion blows the star completely apart without leaving a black hole remnant behind.

Although the SN 2006gy supernova was intrinsically about one hundred times as luminous as SN 1987A, which was bright enough to be seen by the naked eye, SN 2006gy was more than 1,400 times as far away as SN 1987A, and too far away to be seen without a telescope.

Denis Leahy and Rachid Ouyed, Canadian scientists from the University of Calgary have proposed that SN 2006gy was the birth of a quark star. Another possibility is that SN 2006gy is not actually a pair-instability supernova but instead is powered by interaction with a dense circumstellar medium – a Type IIn supernova.

Eta Carinæ (η Carinæ or η Car) is a highly luminous hypergiant star located approximately 7,500 light years from Earth in the Milky Way galaxy. Since Eta Carinæ is 32,000 times closer than SN2006gy, the light from it will be about a billion-fold brighter. It is estimated to be similar in size to the star which became SN2006gy. Dave Pooley, one of the discoverers of SN2006gy, says that if Eta Carinæ exploded in a similar fashion, it would be bright enough that one could read by its light here on Earth nights, and would even be visible during the day time. SN2006gy's Apparent magnitude (m) is 15, so a similar event at Eta Carinæ will have an m of about -7.5. According to astrophysicist Mario Livio, this could happen at any time, but the risk to life on Earth would be low.>>

[owlice]

http://bb.nightskylive.net/asterisk/vie ... 41&t=22025

[neufer]

http://bb.nightskylive.net/asterisk/vie ... 41&t=22025

That's about Allan Sandage; Ann's thread involves Alan Sandage.