Question

types of stars

1. what does a main-sequence star maintain during this stage?
2. what do very massive stars become during the main-sequence phase?
3. what happens to the outer surface of a star during the red giant phase?
4. why does a white dwarf glow?
5. why can a brown dwarf be considered a \failed star\?
6. what is a variable star?
7. what force causes binary star systems to stay together?

Explanation:

These questions are about stellar evolution and properties of stars, which falls under the subfield of Physics (Astronomy) in Natural Science. Let's answer each:

1. A main - sequence star maintains hydrostatic equilibrium (balance between gravitational contraction and thermal pressure from nuclear fusion) and fuses hydrogen into helium in its core.
2. Very massive stars become O - type or B - type main - sequence stars (hot, luminous, and have high mass, fusing hydrogen rapidly).
3. During the red giant phase, the outer surface of a star expands and cools, becoming redder and much larger in size.
4. A white dwarf glows because it is radiating the residual thermal energy from its earlier stellar phases (it has no ongoing nuclear fusion, just cooling from its hot interior).
5. A brown dwarf is a "failed star" because it has too little mass to sustain hydrogen fusion in its core (the mass is below the ~0.08 solar mass threshold needed for hydrogen fusion to start).
6. A variable star is a star whose brightness (luminosity) changes over time, either due to intrinsic changes (like pulsations, eruptions) or extrinsic factors (like eclipses in binary systems).
7. The gravitational force between the two stars in a binary star system causes them to stay together, orbiting around their common center of mass.

Answer:

1. Hydrostatic equilibrium (and hydrogen fusion in core)
2. O - type/B - type main - sequence stars (hot, massive)
3. Expands, cools, becomes larger
4. Radiates residual thermal energy
5. Insufficient mass for H fusion in core
6. Star with changing brightness
7. Gravitational force