A conservative estimate for the upper limit of the mass of a main sequence star
ID: 288884 • Letter: A
Question
A conservative estimate for the upper limit of the mass of a main sequence star is about 150 solar masses. More massive stars do not exist because
A. the strong self-gravity of such stars leads them to collapse directly into black holes.
B. the thermonuclear reactions in such stars proceed so rapidly that the star explodes.
C. such stars contract directly to become planet-like objects.
D. their size is limited by the intensity of the radiation they emit.
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Clusters of stars are important to astronomers because
A. they give astronomers a direct measurement of the age of the Sun.
B. they are the most likely places to contain dark nebulae.
C. the stars in them are about the same mass.
D. the stars in them are about the same age.
E. [Both C and D above.]
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Open clusters of stars
A. include many thousand stars of different ages with different chemical compositions.
B. include many thousand stars of different ages but the same chemical composition.
C. include many thousand stars, all very old, and with very little evidence of elements heavier than helium.
D. include a few hundred stars, often very young and still embedded in the gas and dust from which they were formed.
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H-R diagrams of the youngest star clusters show
A. only a few stars.
B. very few of the most luminous stars.
C. that massive stars have not yet reached the main sequence.
D. that low-mass stars have not yet reached the main sequence.
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Globular clusters are found primarily
A. in the disk of the Galaxy, near the nucleus.
B. in the disk of the Galaxy, far from the nucleus.
C. in the disk of the Galaxy, randomly spaced.
D. outside the disk of the Galaxy.
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Explanation / Answer
1) Answer - A
If the mass of the stellar remnant exceeds 1.4 solar masses then the electron degenerate pressure is insufficient to withstand the force of gravity. The core will thus continue to collapse and form either a neutron star or a black hole.
2) Answer - D
A star cluster is a group of stars that share a common origin and are gravitationally bound for some length of time. They are are particularly useful to astronomers as they provide a way to study and model stellar evolution and ages
3) Answer - D
Open clusters are so-named due to the fact that the individual component stars are easily resolved through a telescope. Some examples such as the Hyades and Pleiades are so close that the individual stars can be clearly distinguished by the naked eye. They are sometimes called galactic clusters due to their location on the dusty spiral arms on the plane of spiral galaxies. Stars in an open cluster have a common origin - they formed from the same initial giant molecular cloud.
They are generally young objects, up to a few tens of millions of years old, with a few rare exceptions as old as a few billion years.
4) Answer - D
We note that there are some galactic clusters that are missing the lower (cooler) end of the main sequence. We think that these clusters are very young. The lower mass stars are still in the process of collapse, and have not yet reached the main sequence.
5) Answer - A
In our galaxy, globular clusters are distributed roughly spherically in the galactic halo, around the galactic centre, orbiting the center in highly elliptical orbits.
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