3.1.6 Stellar Evolution
- A star spends most of its life on the main sequence – it produces energy
through hydrogen to helium fusion
• It slowly becomes hotter and brighter
- It moves off the main sequence as it begins helium fusion (typically becomes
larger, cooler, and brighter)
- Large stars move through stages quickly
• Stars larger than 8 solar masses finish their lives as supernovae
- Stars the size of the sun move into a red giant phase before gently
collapsing into white dwarves
• If they’re a bit larger they can collapse into neutron stars or black holes.
- Small stars (smaller than 5 solar masses) collapse under their own gravity
• They become smaller and temporarily hotter as gravitational energy
converts into heat – white dwarves
• They can also pull nearby material in with their gravitational force,
producing a Type Ia supernova
- Very large stars (between 5-20 solar masses) go through a yellow-supergiant,
Cepheid-variable phase late in their lives
• Variation in brightness is related to absolute magnitude.
3.1.7 A Distance Scale Based on the Intrinsic Brightness of Stars
- From H-R Diagram, we can know how intrinsicly bright a star is.
- By comparing the apparent and absolute magnitudes, we can determine the
distance to the star.
3.1.8 A Distance Scale Based on the Intrinsic Brightness of Galaxies
- For the closest galaxies, we can determine their size if we know their
- Larger galaxies = brighter galaxies
- We measure the distance of galaxies by using the apparent brightness of the
brightest resolvable stars in the galaxy
• We can now measure distances to galaxies out to 10 Mpc
- One of the brightest kinds of stars is the Cepheid variable type
• Extremely large, yellow stars thought to be in the stage of life following
• Polaris (our North Star) is a Cepheid.
- If we can recognize the kind of star, and we know how far away they are, we
can figure out how intrinsically bright they are
- At great distances, we run into another kind of galaxy, the Quasar.
• They produce enormous amounts of light and energy • They are probably enormous young galaxies in formation that have
nearby galaxies or black holes consuming enormous amounts of mass
- Quasars are very far away and very old.
- The extreme brightness of quasars allow us to extend our distance scale out
to the boundaries of our universe (3000 Mpc).
3.1.9 Hubble: “Red-shift” and the “Age” of the Universe
- In the 1920s, astronomers noticed a spectral shift in galaxies that were
• The spectrum tended to shift towards the red end as they appeared
smaller in the field of vision.
- Edwin Hubble used the luminosity-period relationship of Cepheid variable
stars as a means of measuring distance to many galaxies.
• The farther away the galaxy, the more red light it