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
The dust (around newly formed stars) will disappear after about ……….. million years unless the disk is continually supplied with new material.
30
Local comets and asteroids are the most likely sources of new dust. As the planet-size bodies grow, they stir up the orbits of smaller objects in the area. These small bodies collide at high speeds, shatter, and produce tiny particles of silicate dust and ices that can keep the disk supplied with the debris from these collisions
silicate: any of a large number of common minerals formed of silica, oxygen, and one or more other elements.
R 2
Remember that the heavy bombardment in the early solar system ended when the Sun was only about ………… years old.
Observations show that the dusty “debris disks” around stars also become largely undetectable by the time the stars reach an age of ………….. years.
500 million
400 to 500 million
In ………………………, after decades of effort, we found the first such exoplanet (a planet outside our solar system) orbiting a main-sequence star, and today we know that most stars form with planets.
1995
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
Suppose the planet is like Jupiter and has a mass about one-thousandth that of its star; in this case, the size of the star’s orbit is ……………. the size of the planet’s.
one-thousandth
Measuring positions of start and planets orbiting each other in the sky this accurately is extremely difficult, and so far, astronomers have not made any confirmed detections of planets using this technique. However, we have been successful in using …………… to measure the changing velocity of stars with planets around them.
spectrometers
Since that initial planet discovery, the rate of progress has been breathtaking. Almost a thousand giant planets have been discovered using the Doppler technique. Many of these giant planets are orbiting close to their stars—astronomers have called these ……… …….
hot Jupiters
So if such planets exist (hot Jupiters - shortest time to complete their orbit.), we would expect to be finding this type first.
Scientists call this a ………….—where our technique of discovery selects certain kinds of objects as “easy finds.”
selection effect
