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Question 1

An interesting property of brown dwarfs is that they are all about the same radius as …………., regardless of their masses. Amazingly, this covers a range of masses from about ………. to …….. times the mass of ……………. (M_J).

Answer 1

Jupiter / 13 80 / Jupiter

Question 2

So, what is the difference between a low-mass brown dwarf and a high-mass planet?

The International Astronomical Union considers the distinctive feature to be ………. ……….

Answer 2

deuterium fusion

Question 3

If an object has enough mass to fuse deuterium (about …….. M_J or ………… M_Sun), it is a brown dwarf.

Objects with less than ………. M_J do not fuse deuterium and are usually considered planets.

Answer 3

13 0.012

13

Question 4

stars come in a wide variety of sizes. At some periods in their lives, stars can expand to enormous dimensions. Stars of such exaggerated size are called …………..

Answer 4

giants

Question 5

Suppose you want to determine whether a star is a giant. A giant star has a large, extended ……………

Answer 5

photosphere

Question 6

Because large photosphere mean atoms are spread without pressure this leads to results:

1. ………… ……….. ………
2. and the atoms stays ionized, why?

Answer 6

1. narrower spectral lines
2. due to the space between, electrons flying around are unlikely collide with ionized atoms

Question 7

Identifying spectral line required:

1. identifying the same spectral line on elements on earth
2. Sometimes nature is unhelpful, and lines of different elements have …………. …………,
3. …………. of the star can change the observed wavelength of each of the lines.

Answer 7

1. FYI
2. identical wavelengths
3. motion

Question 8

Studies of stellar spectra have shown that hydrogen makes up about ……………………. of the mass of most stars. Helium is the second-most abundant element, making up almost a …………… of a star’s mass. Together, hydrogen and helium make up from 96 to 99% of the mass; in some stars, they amount to more than 99.9%.

Answer 8

three-quarters / quarter

Question 9

Generally, but not always, the elements of ……….. atomic weight are more abundant than those of …………. atomic weight.

Answer 9

lower / higher

Question 10

The fraction of a star’s mass that is composed of these elements is referred to as the star’s ………………

The ……………. of the Sun, for example, is 0.02, since 2% of the Sun’s mass is made of elements heavier than helium.

Answer 10

• metallicity*
• astronomers use the term “metals” to refer to all elements heavier than hydrogen and helium.*

Question 11

…………. …………., pioneering yet again, in 1868 made the first radial velocity determination of a star.

Answer 11

William Huggins

Question 12

To know the true ……….. ……… of a star—that is, its total speed and the direction in which it is moving through space relative to the Sun—we must know its ……….. …………, ……….. ……….., and …………..

Answer 12

space velocity

radial velocity / proper motion / distance

Question 13

when star rotates, we receive both light from the side rotating toward us and the rotating away from us and because it is far a way both spectrum gets merged into one. by doing so the spectral lines of both sides get merged and become …………. than normal

Answer 13

wider

Question 14

In fact, astronomers call this effect (doppler effect of rotating star) ……….. …………, and the amount of broadening can tell us the ………… at which the star rotates

Answer 14

line broadening / speed

Question 15

Measurements of the widths of spectral lines show that many stars rotate faster than the Sun, some with periods of less than a day! These rapid rotators spin so fast that their shapes are “flattened” into what we call ………… ………..

Answer 15

• oblate spheroids*
• An example of this is the star Vega, which rotates once every 12.5 hours. Vega’s rotation flattens its shape so much that its diameter at the equator is 23% wider than its diameter at the poles*

Question 16

Studies have shown that stars decrease their rotational speed as they age. Young stars rotate very quickly, with rotational periods of days or less. Very old stars can have rotation periods of several months.

Answer 16

R 2