21 (3) Flashcards
Objects of extremely low mass never achieve high-enough central temperatures to ignite nuclear reactions. The lower end of the main sequence stops where stars have a mass just barely great enough to sustain nuclear reactions at a sufficient rate to stop gravitational contraction.
R 2
This critical mass (mass enough to sustain nuclear reactions) is calculated to be about ………… times the mass of the Sun.
0.075
At the other extreme, the upper end of the main sequence terminates at the point where the energy radiated by the newly forming massive star becomes so great that it halts the accretion of additional matter. The upper limit of stellar mass is between ………. and ………. solar masses.
accretion: a gradual increase or growth by the addition of new layers or parts:
100 / 200
A disk of gas and dust appears to be an essential part of ………. ……….. Observations show that nearly all very young protostars have disks and that the disks range in size from …….. to ………. AU.
star formation
10 / 1000
The mass contained in these disks is typically …………. of the mass of our own Sun, which is more than the mass of all the planets in our solar system put together. Such observations already demonstrate that a large fraction of stars begin their lives with enough material in the right place to form a planetary system.
1–10%
if a protostar is less than about …………. million years old, its disk extends all the way from very close to the surface of the star out to …………… of AU away.
1 to 3 / tens or hundreds
In older protostars, we find disks with outer parts that still contain large amounts of dust, but the inner regions have lost most of their dust. In these objects, the disk looks like a …………, with the protostar centered in its hole.
donut
The inner, dense parts of most disks have disappeared by the time the stars are ………… …….. years old
10 million
Calculations also show that any small dust particles and gas that were initially located in the region between the protostar and the planet, and that are not swept up by the planet, will then fall onto the star very quickly in about ………. years.
50,000
If the formation of a planet is indeed what produces and sustains holes in the disks that surround very young stars, then planets must form in…………. million years. This is a short period compared with the lifetimes of most stars and shows that the formation of planets may be a quick byproduct of the birth of stars.
3 to 30
…………… (the small chunks of solid matter—ice and dust particles)
planetesimals
If the growing planets reach a mass bigger than about 10 times the mass of Earth, their gravity is strong enough to capture and hold on to ……….. …….. that remains in the disk.
hydrogen gas
At that point (planet 10 times the mass of Earth), they will grow in mass and radius rapidly, reaching ………. …….. ……… However, to do so requires that the rapidly evolving central star hasn’t yet driven away the gas in the disk with its increasingly vigorous wind
giant planet dimensions
From observations, we see that the disk can be blown away within ……………. years, so growth of a giant planet must also be a very fast process, astronomically speaking.
10 million
The dust around newly formed stars is gradually either incorporated into the growing planets in the newly forming planetary system or …………. through gravitational interactions with the planets into ……….
ejected / space
bear in mind that the …………. method—which relies on the pull of a planet making its star “wiggle” back and forth around the center of mass—is most effective at finding planets that are both close to their stars and massive.
Doppler-shift
