Part 18: Classification and properties of galaxies.
These short videos were created in August 2007 by Dr. Christopher D. Impey, Professor of Astronomy at the University of Arizona, for his students. They cover a broad range of terms, concepts, and princples in astronomy and astrobiology. Dr. Impey is a University Distinguished Professor and Deputy Head of the Astonomy Department. All videos are intended solely for educational purposes and are licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. The full list of collections follows below:
01. Fundamentals of Science and Astronomy
02. Ancient Astronomy and Celestial Phenomena
03. Concepts and History of Astronomy and Physics
04. Chemistry and Physics
05. Quantum Theory and Radiation
06. Optics and Quantum Theory
07. Geology and Physics
08. Solar Neighborhood and Space Exploration
09. Outer Planets and Planetary Atmospheres
10. The Solar System
11. Interplanetary Bodies
12. Formation and Nature of Planetary Systems
13. Particle Physics and the Sun
14. Stars 1
15. Stars 2
16. Stars 3
17. Galactic Mass Distribtuion and Galaxy Structure
19. Galaxies 2
20. Galaxy Interaction and Motion
21. Deep Space and High-Energy Phenomena
22. The Big Bang, Inflation, and General Cosmology
23. The Big Bang, Inflation, and General Cosmology 2
24. Chemistry and Context for Life
25. Early Earth and Life Processes
26. Life on Earth
27. Life in the Universe
28. Interstellar Travel, SETI, and the Rarity of Life
29. Prospects of Nonhuman Intelligences
Transcript: Stars of a particular photospheric temperature can have vastly different sizes and luminosities in an HR diagram. For example at a temperature of about three thousand Kelvin there is Proxima Centauri, a low mass main sequence star only three percent the size of the Sun, Aldebaran, that’s twenty times larger than the Sun, and Betelgeuse, a red supergiant a thousand times the Sun’s size. Yet spectroscopy alone can distinguish between these situations. This is because spectral lines reveal the diffuse nature of the atmosphere. In the more diffuse atmospheres of the larger stars there are fewer collisions, and the lines are narrower. In the denser atmospheres of the smaller stars there are more collisions, and the lines are broader so line broadening gives an information about luminosity class and astronomers can distinguish between main sequence stars, giants, and supergiants.
Distances From Cepheid Variables
Transcript: Cepheid variables are luminous stars with variations in a range of periods of one to fifty days. The physics of their pulsation is well understood, and empirically for stars with well measured distance by parallax, there’s a well determined relationship between the period of the pulsation and the luminosity of the star. More luminous Cepheids have longer periods. Astronomers therefore isolate Cepheids in a distant cluster by taking images over a period of several months to identify the variable stars and measure their periods. The period then leads to a prediction of the luminosity. That is combined with the apparent brightness to yield a distance. The Cepheid distance measurement technique is among the most accurate in astronomy with a precision of ten percent.
Distances and Rare Stars
Transcript: Main sequence fitting can be applied in principle to any cluster. However, the rare variable stars like RR Lyraes and Cepheid variables are particularly valuable because the physics of their variations allows their luminosities to be estimated, and their luminosities allow them to be seen to large distances. RR Lyraes are a hundred times more luminous than the Sun, so they can be seen ten times further away than a Sun-like star could. Cepheid variables are ten thousand times more luminous than the Sun and so can be seen at distances a hundred times that of a Sun-like star. But we really need a large cluster to be able to detect even a few versions of these very rare stars, and that is a practical limitation.
Distance and Obscuration
Transcript: In the early part of the twentieth century, astronomers calculated the distances to stars by assuming that interstellar space was perfectly transparent. But eventually comparisons of distance to clusters in different directions in the sky yielded inconsistent results, and in 1930 Robert Trumpler showed that interstellar extinction or obscuration dims the light from all stars, groups, and clusters, that are larger than a distance of a few dozen parsecs. What this means is that the intensity of light falls off more rapidly than would be predicted by the inverse square law. We see a star as dimmer than it truly is, and we overestimate its distance. Without taking into account interstellar obscuration it’s impossible to correctly measure distances to stars, groups, and clusters, and map out the Milky Way galaxy.
Ages of Star Clusters
Transcript: The ages of stars are derived from stellar models. The physics is complex so computers are used to simulate energy transport mechanisms. The details depend on heavy element abundance and on the mechanism for helium diffusion in the atmosphere of the stars. Thus there are uncertainties attached to the prediction of luminosity from stellar models. There are also uncertainties attached to the determination of luminosity from observation of stars. This can include the effect of interstellar extinction and uncertainties in the distance estimates. Since luminosity is proportional to distance squared, a ten percent error in the distance leads to a twenty percent error in luminosity. For all of these reasons it’s difficult to measure ages more accurately than ten or twenty percent.
Main Sequence Turnoff
Transcript: The properties of stars in a star cluster as measured in the HR diagram change with time, and this can be a chronometer for measuring the age of groups of stars. The main sequence for a young star cluster is fully populated all the way up to the most massive, most luminous, and hottest stars. Remember that the main sequence runs from high luminosity and high temperature and high mass down to low luminosity, low temperature, and low mass. After ten to the seven years stars more than a thousand times the luminosity of the Sun have left the main sequence. After ten to the eight years stars more than a hundred times the luminosity of the Sun have left the main sequence. After ten to the nine years stars more than five times the Sun’s luminosity have left the main sequence, and after ten to the ten or ten billion years a star like the Sun is leaving the main sequence. Stars much less massive than the Sun have not had long enough in the age of the universe to exhaust their hydrogen, and so the main sequence is always populated in any star cluster for very low mass stars. This evolving point at which the most massive most luminous and hottest star exist on the main sequence is called the main sequence turnoff point.