Stars, 30 October 2013!To calculate the inherent brightness of a star you have to compare how bright 1.it looks from Earth to how far away it is.! Brightness decreases with the square of the distance. !A.It is easy to measure the light of stars from earth. !a.Distance is difficult to calculate in astronomy, and becomes less accurate b.as objects get farther away.!If a star is close enough, astronomers can triangulate to it or use 1.estimates from the Hipparcos satellite. !The Hipparcos satellite is the source of the best data on distances A.about 50-100 light years away: !It was created by the European space agency in the a.1980s-1990s.!It increased accuracy at measuring distances tenfold, and b.showed that a quarter of astronomers' previous estimates were wrong.!A quarter that were thought to be within 25 light years were 1.farther, and a quarter thought to be farther than 25 light years were closer.!If the star is farther than a couple hundred light years the estimate of 2.its distance is an informed guess. !The formal margin of error is 15%, but it could be 30%, or pure A.fiction.!Being off by a quarter million light years has happened:!a.There was a whole category of objects in the sky treated as 1.stars that were actually the nuclei of galaxies a few hundred million light years away. !This is an age old problem of Astronomy:!3.Herschel was working on this problem in the late 1700s. !A.Positional telescopes were good enough to get the first accurate B.star distances in 1838. !By 1910, astronomers had estimates for the distance of about a.100 stars.!Stars have occupied the attention of humans in general for as long C.as they have had the intellect to wonder about them.!Constellations are groups of stars that humans associate a.together.!For example, Orion, Taurus, Andromeda.!1.This is not scientific: Stars associated with each other in 2.constellation aren't necessarily associated with each other in the sky: !You cannot tell which stars are near and what is far by A.looking at a star field. !Though Aldebaran, the Pleiades star cluster, and the B.Hyades star cluster are all in the constellation Taurus they range from 68 light years away to 415 light years away.!NOTE: In the last 30 years, there are companies that claim to be b.able to name a star for you or your family member. They do not have this authority, and are scamming you. !For example, the International Star Registry. !1.They are still advertised in the media because it cares more 2.about ad revenue than serving the interests of their customers. !The closest stars to Earth vary in brightness:!B.It was a huge surprise to find a lot of the faint stars in the sky were a.actually very close, and a lot of the brightest stars were very far away. !Stars within 12 light years: bright stars, medium bright stars, a lot of red b.dwarves!Red dwarves are the most abundant kind of star where astronomers 1.can detect them. !Astronomers extrapolate that they are abundant throughout the A.universe in general, although they are so faint that only those within a few hundred light years can be detected. !Problem: This part of the universe might not be representative a.of all parts. !Because they are inherently faint, red dwarves attract much less B.attention than they deserve.!By finding the peak emissions of stars, astronomers can determine their 2.surface temperatures. !By the early 1800s, astronomers had telescopes good enough to notice A.tinges of color on the stars: !A lot of stars that have contrasting colors are pretty close together. !a.For example, the Alberio double star has a blue star and a yellow star. !1.This was recognized as an important clue to the nature of stars from the B.work in physics by Max Planck: !By investigating the spectrum of materials at different temperatures, he a.discovered that the overall amount of light given by objects depends mostly on their temperature rather than what they are made of. !In general, the hotter an object is, the more light it gives off at all C.wavelengths than cooler objects and the shorter the wavelength of its peak emission. !An object that is 2000 degrees kelvin puts out mostly infrared light. !a.An object that is 3000 degrees kelvin gives off more light at all b.wavelengths, and its peak is shifted towards the shorter end of the spectrum. !An object that is 4000 degrees kelvin gives off more light at all c.wavelengths and the peak of its emission is in the red part of the visible part of the spectrum. !The previous cooler objects have a reddish tinge. !1.An object that is 5000 degrees kelvin gives off more light at all d.wavelengths and the peak of its emission is in the yellow part of the visible spectrum. !Hotter objects like the Sun, Sirius, Rigel, and the red giant star e.Betelgeuse have peak emissions in the ultraviolet. !Astronomers have to subtract the effect of water vapor in the Earth's D.atmosphere when doing this analysis from ground-based observatories: !The water vapor absorbs infrared radiation, causing absorption lines in a.spectra.!Classifying stars by their spectra: OBAFGKM!3.In the late 1800s, most observatories were occupied with the positions and A.movement of stars. !For example, accurately noting the right ascension and declination of a.stars. !When Charles Pickering became the director of the Harvard observatory, B.he shifted the focus to astrophotography and spectroscopy. !He believed finding out the characteristics of stars would be the future a.of astronomy, and he was right. !By 1881, Pickering was having spectra photographed by night and analyzed C.by day. !To be able to afford a 24 hour shift of astronomers, he hired more than a.20 women to analyze the data generated each night.!In this period, women were getting college educations, but there was 1.hardly any scientific work for them. !A lot of their work involved computing, so they were called 2."computers."!They were also called Pickering's Harem.!A.Some of these women just did the work for a while, and others 3.became distinguished astronomers.!The first female computer Pickering hired was a house keeper 4.Williamina Fleming. !She was trained as a school teacher in Scotland, but she was A.desperate for work after immigrating to the US. !The story is that Pickering, dissatisfied by the inferior work of a B.computer, claimed that his house keeper could do better. He hired her, and she was in charge of the project within a year. !He then hired more women. !a.She had a distinguished career in
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