Test 2 Flashcards
Phases during stellar evolution
I. Collapse of a gas cloud to form a star (book stages 1-6)
II. Main Sequence (stage 7)
III. Red Giant
IV. Helium Flash
V. Second Red Giant (Asymptotic Giant Branch [AGB])
VI. Planetary Nebula and formation of White Dwarf
It is helpful to keep in mind that stars are a battle between
gravity and nuclear energy. And gravity always wins
What is the main sequence?
The energy of a main sequence star is produced by fusion of H to He in the star’s core.
What is hydrostatic balance
Recall that stars are a
balance between pressure
pushing stuff out and
gravity pulling stuff in.
Because stars are constantly losing energy, they would contract if it were not for a heat source (nuclear fusion) in the center
What is the Red giant phase?
Eventually the star gets hot enough to burn hydrogen in the outer layer around the He core. This is called hydrogen shell burning.
This is the Red Giant
stage
Why is a red giant red?
Because of the vast expansion, the photosphere
of a red giant is cooler - and therefore redder -
than that of the main sequence star from which it
evolved.
As the shell around the core burns, more helium
is added to the core, which contracts faster and
makes the shell still hotter, so the star keeps
getting larger and brighter.
What happens during stages 8 and 9
- The inner core has fused to helium and begins to contract
and heat up. - The core is now hot enough to drive a furious rate of fusion
of H to He in a shell around the core. The outer part of the
star vastly expands from the combined energy of contraction
and furious shell burning. The core continues to contract and
heat up.
The red giant stage ends when the core is finally hot enough
at 100 million K to fuse helium to carbon.
Not all stars get to this point, which depends on
Helium Flash (stage 9)
For stars with M > 0.4 Msun, the temperature will reach 100
million K, hot enough to ignite the fusion of He in the core to
produce C.
Three helium nuclei combine to form a carbon nucleus:
4He + 4He + 4He → 12C
Fusion of helium to carbon begins explosively throughout the
core, and the burst of energy released is called the helium
flash. The core re-expands, the H burning in the shell slows
because the shell is now cooler, and the outer layers of the
star contract.
In an H-R diagram, stars with the smallest radius are found in the ___________ of the diagram
lower left corner
We know that giant stars are larger in diameter than the sun because
they are more luminous but have about the same temperature.
The most common stars are
lower main sequence stars
less luminous
The ____________ of a star is a measure of the total energy radiated by the star across all wavelengths in one second.
Luminosity
To determine the period of a visual binary, we must measure
position
Spectroscopic binaries are difficult to analyze because
We can’t see the shape of the orbit
Stars on the main sequence with the greatest mass
are spectral type O stars
An eclipsing binary will
always be a spectroscopic binary
In the HR diagram, 90% of all stars are
on the main sequence
Which star in the diagram is most like the sun?
HR 5337
Which star in the HR diagram above has the greatest surface temperature?
Alnilam
Which of the stars in the HR diagram above has the largest absolute visual magnitude?
Sirius B
To measure the parallax of the most distant stars measurable, we would make two measurements of the star’s position on the sky separated by
6 months
Parallax is used to measure a star’s
distance
How is the distance to a star related to its parallax?
Distance is inversely proportional to parallax
A parsec is measure of
size and distance
Star A and star B appear equally bright, but star A is twice as far from us as star B. Which of the following is true?
Star A is four times as luminous as star B
Star A and star B both have the same temperature, but different sizes and distances. As a result, star A is more luminous than star B, but star B is brighter than star A. Which of these statements about the absolute and apparent magnitudes of the two stars is correct?
Star A has a larger apparent magnitude, while star B has a larger absolute magnitude
Stars that have spectral type B _________ in temperature compared with stars that have spectral type M.
are hotter
Star A is a red star. Star B is a blue star. Which star is hotter?
Star B
Two stars with similar temperatures but different sizes will have
similar spectral types but different luminosities
Stars are mostly made of
Hydrogen
Star C is a red star. Star D is a blue star. Which has a larger radius?
We also need to know the luminosities of the stars to determine their radii
Star E is the same temperature as star F, but star E is four times as luminous as star F. How to the radii of the stars compare?
The radius of star E is twice that of star F
