Astronomy #8
Describe the helium flash and why it occurs.
A helium flash is an explosion that happens to a low-mass star after it gets off the main sequence. In the core of the star, helium fusion occurs and creates degenerate electrons. Once the helium fusion is ignited, the core heats up very quickly and expands. This increases luminosity in the star.
What is the typical age for a globular cluster associated with our Milky Way?
10-12 billion years
What temperature is needed to fuse helium into carbon?
100 million K
Why don't stars live forever? Which stars live the longest?
Because every star reaches a point where it can no longer burn hydrogen in its core, thus gradually increasing its luminosity and radius until the point of 'extinction', if you will. Red dwarf stars live the longest because they burn their hydrogen slower than other stars.
What inevitably forces a star like the Sun to evolve away from being a main sequence star?
Helium builds up in the core, while the hydrogen burning shell expands.
What is the region of the Hertzsprung-Russell diagram known as the horizontal branch?
It is a region of the H-R diagram, where stars remain for a time as they undergo helium core fusion.
A star is on the horizontal branch of the H-R diagram. Which statement is true?
It is burning both hydrogen and helium.
In the evolution of massive stars, what is the significance of the temperature 600 million K?
It is the temperature needed for carbon fusing into heavier elements.
What spectral type of star that is still around formed longest ago?
M
At which stage in a Sun-like star's life is its core the least dense?
Main Sequence
What spectral type of star that is still around formed most recently?
O
Mass transfer in binaries occurs when one giant swells to reach the:
Roche Lobe.
Which of the following elements contained in your body is NOT formed in the cores of stars during thermonuclear fusion?
hydrogen
When a low-mass star runs out of hydrogen in its core, it gets brighter. Why?
The core has a contracting effect that raises the star's temperature and pushes its hydrogen burning shell outwards.
As a low-mass main-sequence star runs out of hydrogen fuel in its core, it actually becomes brighter. How is this possible?
The outer layers expand due to the higher rate of fusion in a shell around the dead core.
Will the Sun ever turn into a brown dwarf? Why or why not?
The sun is huge and is able to undergo nuclear fusion to get on the main sequence, which a brown dwarf cannot do. So no, the Sun will not turn into a brown dwarf.
How do astronomers explain the existence of "blue stragglers"?
They are main-sequence stars whose evolution has been altered by mergers or binary companions, causing them to live longer than usual for a star of their spectral type.
Why don't stars live forever?
They run out of nuclear fuel in their cores.
How is the age of a star cluster determined?
We know that all of the stars in the cluster were formed roughly the same time. In order to determine the age, we can find the main-sequence-turnoff point.
Compared to our Sun, a typical white dwarf has:
about the same mass and a million times higher density.
What is the size of the core of a typical white dwarf star?
about the size of Earth
A star (no matter what its mass) spends most of its life:
as a main sequence star.
On the main sequence, massive stars
burn their hydrogen fuel more rapidly than the Sun.
What are black dwarfs?
cooled off white dwarfs that no longer glow visibly
Compared to other stars on the H-R diagram, red-giant stars are so named because they are
cooler
A 20 solar mass star will stay on the main sequence for 10 million years, yet its iron core can exist for only a:
day
A white dwarf is supported by the pressure of tightly packed
electrons
Hydrogen-shell burning proceeds increasingly faster due to which of the following?
heat released from the core's contraction
When a star's inward gravity and outward pressure are balanced, the star is said to be:
in hydrostatic equilibrium.
The element with the most stable nucleus and smallest mass per particle is:
iron
The Roche lobe of a star in a binary star system:
is, in terms of the star's gravity, its "zone of influence."
A high-mass star dies more violently than a low-mass star because:
it generates more heat and its core eventually collapses very suddenly.
Virtually all the carbon-rich dust in the plane of the galaxy originated in:
low-mass stars.
What can disrupt the evolutionary sequence of a star?
mass transfer can affect the evolutionary sequence of a star.
For a nova to occur, the system must have already been a(n):
mass-transfer binary.
In neutronization of the core, a proton and an electron make a neutron and a(n):
neutrino
A surface explosion on a white dwarf, caused by falling matter from the atmosphere of its binary companion, creates what kind of object?
nova
Which of these will the Sun probably become in the very distant future?
planetary nebula
The "helium flash" occurs at what stage in stellar evolution?
red giant
In globular clusters, the brightest stars will be:
red supergiants.
What is a planetary nebula?
the ejected envelope, often bipolar, of a red giant surrounding a stellar core remnant
What characteristic of a star cluster is used to determine its age?
the main sequence turnoff
During the hydrogen shell burning phase:
the star grows more luminous.
When a stellar iron core collapses, large numbers of neutrinos are formed, and then:
they immediately pass through the core and escape to space.
When the Sun is on the red-giant branch, it will be found at the
upper right of the H-R diagram.
The order of evolutionary stages of a star like the Sun would be Main Sequence, giant, planetary nebula, and finally:
white dwarf
A star like the Sun will end up as a
white dwarf.
