The H-R Diagram Image credit: NOAO
Luminosity, Temperature, Radius Recall: apparent magnitude, absolute magnitude, and distance are related. We now have a method for finding the luminosity and a method for finding the surface temperature of a star. With these two pieces of information, we can now determine the radius (or diameter) of the star. Relationship between: luminosity, temperature, and radius
Where are the largest stars? A B C D
Where are the smallest stars? A B C D
Stellar Sizes
Stellar Sizes
Stellar Sizes
Luminosity, Temperature, Radius The Hertzsprung-Russell diagram plots luminosity vs. temperature. Indirectly, it also shows size.
Stellar Spectra At the time of Cannon s work on stellar spectra, no one understood why the OBAFGKM order was correct. Many astronomers thought that each spectral type was due to different chemical compositions. The natural assumption: the stars were made of the same stuff as Earth: rock and metal. The only difference was stars were hotter! Cecilia Payne overturned this view by combining two emerging fields of study: the work on spectra at Harvard and the study of atoms.
Stellar Spectra Cecilia Payne used research by Megh Saha to find that spectra were controlled by temperature. Payne proved why Cannon s order worked. She also found that hydrogen and helium were the most abundant elements in the Sun & stars. Princeton professor Henry Norris Russell reviewed her calculations and claimed her results were clearly impossible. She concluded that her analysis was correct but almost certainly not real. She was vindicated several years later and won the prestigious H. N. Russell Prize in 1977.
What is a Star? Stars are born with a composition of (roughly): 75% hydrogen 25% helium 1-2% all other elements (mostly carbon, oxygen) During most of their lives (as main sequence stars), stars fuse hydrogen into helium.
Size vs. Mass Consider a kilo of feathers and a kilo of lead. Which has a higher density? Which contains more mass? Which naturally fills a larger space? We have to be just as careful when describing the size and the mass of stars. Large stars are not necessarily high-mass stars!
Size vs. Mass Conversely, white dwarfs are Earth-sized but they have about as much mass as the Sun! What can we say about white dwarfs densities? We will return to the question of finding a star s mass in a few weeks when we discuss gravity. For now, there is a simple relationship between mass and spectral type for all main sequence stars (and main sequence stars only!).
Mass and the Main Sequence Hottest dwarfs (O & B) are the most massive Coolest dwarfs (K & M) are the least massive
Which of these main-sequence spectral types is most luminous? A. A-type B. B-type C. K-type D. M-type
Stellar Lifetimes Knowing the masses of main-sequence stars, we can begin to understand how long stars last. fuel lifetime = = "burn rate" mass luminosity Massive stars have more fuel, but they also use it up much faster: A star with a mass of 10 times the Sun has a luminosity of 10,000 times the Sun. mass 10 lifetime = = = luminosity 10,000 0.001 of Sun
Stellar Lifetimes Detailed calculations of the Sun s interior indicates the Sun s main sequence lifetime = 10 billion years (a very good number to memorize) Based on the previous calculation, a 10 solar-mass star has a lifetime of: 0.001 x 10 billion years = 10 million years! Stars of much smaller mass have very long lives a 0.1 solar-mass star will fuse hydrogen to helium for 10 trillion years!
Consider these three stars: Ford is a G-type giant star. Trillian is a B-type supergiant star. Zaphod is an K-type main sequence star. Which star has the hottest surface temperature? A. Ford B. Trillian C. Zaphod D. Insufficient information
Consider these three stars: Ford is a G-type giant star. Trillian is a B-type supergiant star. Zaphod is an K-type main sequence star. Which star has the highest luminosity? A. Ford B. Trillian C. Zaphod D. Insufficient information
Consider these three stars: Ford is a G-type giant star. Trillian is a B-type supergiant star. Zaphod is an K-type main sequence star. Which star has the longest remaining lifetime? A. Ford B. Trillian C. Zaphod D. Insufficient information
Consider these three stars: Ford is a G-type giant star. Trillian is a B-type supergiant star. Zaphod is an K-type main sequence star. Which star s color is closest to our Sun s color? A. Ford B. Trillian C. Zaphod D. Insufficient information
Consider these three stars: Ford is a B-type giant star. Trillian is a G-type supergiant star. Zaphod is an K-type main sequence star. Which star is the closest to Earth? A. Ford B. Trillian C. Zaphod D. Insufficient information