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
Which of the following is NOT evidence of the existence of an interstellar medium?
molecular absorption lines in the spectra of cool stars
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Compared with the spectral lines in the solar spectrum, lines in a supergiant star’s spectrum are
more narrow
The spectra of most galaxies tell us that
most galaxies appear to be moving away from us
Where are elements heavier than iron primarily produced?
Supernovae
The explosion of a supernova typically leaves behind
a shell of
hot, expanding gas with a pulsar at the center.
After what evolutionary stage does a star become a white dwarf?
Giant
Stars that have ejected a planetary nebula eventually become
white dwarfs.
What nuclear fusion mechanism does an isolated white dwarf use to generate energy?
White dwarfs don’t generate their own energy.
What are the two longest stages in the life of a one solar mass star?
main sequence, white dwarf
For a star of Sun-like mass, what is the last stage of the nuclear fusion?
helium to carbon and oxygen
About how long will a 0.5 star spend on the main sequence?
57 billion years
A planetary nebula has a radius of 0.5 pc and is expanding at 20 km/sec. What is the approximate age of this planetary nebula? (1 pc is equal to 3×1013 km, and 1 year is equal to 3.15×107 seconds.)
24,000 years
The diagram below shows a light curve from a supernova. How many days after maximum light did it take for the supernova to decrease in brightness by a factor of 100?
150
As a white dwarf cools its radius will not change because
pressure does not depend on temperature for a white dwarf because the electrons are degenerate.
If the theory that novae occur in close binary systems is correct, then novae should
repeat after some interval.
What does a planetary nebula produce?
an emission spectrum
A white dwarf is composed of
carbon and oxygen nuclei and degenerate elections
When material expanding away from a star in a binary system reaches the Roche surface
the material is no longer gravitationally bound to the star
A nova is almost always associated with
a white dwarf in a close binary system.
A planetary nebula is
the expelled outer envelope of a medium mass star.
The lowest mass stars cannot become giants because
they cannot heat their centers hot enough.
If the stars at the turnoff point of a cluster have a mass of 3 M, what is the age of the cluster?
6.4x10^8 years
Stars in a star cluster all ____________
have the same age
have the same chemical composition
Stars in a star cluster don’t have
the same Mass
Helium flash occurs
because degenerate electrons in the core do not allow the core to expand as it heats up.
star clusters are important to our study of stars because
they give us a method to test the theories and models of stellar evolution
The Crab nebula is
a supernova remnant
_______is a form of electromagnetic radiation produced by rapidly moving electrons spiraling through magnetic fields.
Synchrotron radiation
Stars within a cluster that are at the turnoff point
have life expectancies that are equal to the age of the cluster
A(n)______ is a collection of 105 to 106 stars in a region 10 to 30 pc in diameter. The stars in the collection tend to be more than 109 years old and mostly yellow and red stars.
globular cluster
A(n) _______ is a collectin of 100 to 1000 stars in a region about 25 pc in diameter. The stars in the collection are typically quite young.
Open cluster
What do you need to form stars
ionized gas in the Intercellular medium
Shortwave light(blue) gets ___________ than long wavelight (red)
scattered more
Stars form in regions that are
coldest and densest
Absolute visual magnitude is
the apparent magnitude of a star observed from a distance of 10 pc.
A star’s absolute magnitude depends only on the star’s
Temp and diameter
In an H-R Diagram, stars with the smallest radius are found in the _______________ of the diagram.
Lower left corner
In the H-R diagram, 90 percent of all stars are
On the main sequence
We know giant stars are larger in diameter than the sun because
they are more luminous but have about the same temperature.
Giant stars
I, II, III, & IV
The most common stars are
lower main sequence stars
The _______________ of a star is a measure of the total energy radiated by the star in one second
Luminosity
What is the distance to a star that has a parallax angle of 0.5 arc seconds?
Two parsecs
At what distance must a star be to have its apparent magnitude equal to its absolute magnitude?
10 parsecs
How can a cool star be more luminous than a hot star?
It can be more luminous if it is larger.
To which luminosity class does the mass-luminosity relationship apply?
The main sequence
Which luminosity class has stars of the lowest density, some even less dense than air at sea level?
The supergiant
In a given volume of space the Red Dwarf (or lower main sequence) stars are the most abundant, however, on many H-R diagrams very few of these stars are plotted. Why?
They have very low luminosity and are difficult to identify
