18 (1) Flashcards
(The Milky Way Galaxy, in which we live, is about ……………. light-years in diameter,
100,000
You can see that there are many more low-luminosity (and hence ……….. ……..) stars than high-luminosity ones in our very local neighborhood, one that contains a tiny fraction of all the billions of stars in the Milky Way.)
low mass
The reason the lowest-mass dwarfs are so hard to find is that they put out very little light—ten thousand to a million times less light than the Sun. Only recently has our technology progressed to the point that we can detect these dim, cool objects.
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The nearest of these three stars, ………… ………….., still cannot be seen without a telescope because it has such a low luminosity.
Proxima Centauri
You should expect the number of known stars within …….. ……………… of the Sun to keep increasing as more and better surveys are undertaken.
21 light-years
We are doing census in out local neighborhood in the milky way
What are the two problems when we do census of the starts in our local neighborhood like we do for people?
- it is hard to be sure we have counted all the inhabitants;
- our local neighborhood may not contain all possible types of (stars) like people.
For example, a few people do live to be over 100 years old, but there may be no such individual within several miles of where you live.
A clue that we are missing something in our stellar census comes from the fact that only six of the 20 stars that appear brightest in our sky—…………………………..—are found within 26 light-years of the Sun
- Sirius,
- Vega,
- Altair,
- Alpha Centauri,
- Fomalhaut, and
- Procyon
Why are we missing most of the brightest stars when we take our census of the local neighborhood (only six of the 20 stars that appear brightest in our sky)?
The answer, interestingly enough, is that the stars that appear brightest are not the ones closest to us.
it turns out that most of the stars visible without a telescope are ………….. of light-years away and many times more luminous than the Sun.
hundreds
The most luminous of the bright stars listed in emit more than ……….. times more energy than does the Sun. These highly luminous stars are missing from the solar neighborhood because they are very rare.
50,000
Selection Effect is the bias introduced when a methodology, respondent sample or analysis is biased toward a specific subset of a target population. Meaning it does not reflect the actual target population as a whole.
You test overall brand awareness of your health food products and decide to collect data via in-person interviews at gyms and health stores. In this example, the data is biased because your methodology is targeting people who frequent health-related venues and therefore are likely predisposed to health food products. This will likely over-state the overall brand awareness of your health food products.
The contrast between these two samples of stars, those that are close to us and those that can be seen with the unaided eye, is an example of a selection effect. When a population of objects (stars in this example) includes a great variety of different types, we must be careful what conclusions we draw from an examination of any particular subgroup.
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The mass of a star—how much material it contains—is one of its most important characteristics. If we know a star’s mass, as we shall see, we can estimate how long it will shine and what its ultimate fate will be.
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Luckily, not all stars live like the Sun, in isolation from other stars. About ………. the stars are binary stars—two stars that orbit each other, bound together by gravity.
Masses of binary stars can be calculated from measurements of their …………, just as the mass of the Sun can be derived by measuring the …………. of the planets around it
half
orbits / orbits
Astronomers call any pair of stars that appear to be close to each other in the sky ………… ………., but not all of these form a true binary, that is, not all of them are ………… ………….
Some are just chance alignments of stars that are actually at different distances from us.
double stars / physically associated
Although stars most commonly come in pairs, there are also ……… and ……….. systems.
triple / quadruple
One well-known binary star is …………, located in the constellation of ……………
Castor / Gemini
(We use the term “…………..” to mean a member of a binary star system.)
component
What was actually the first evidence that gravitational influences exist outside the solar system.
By 1804, astronomer William Herschel, who also discovered the planet Uranus, had noted that the fainter component of Castor had slightly changed its position relative to the brighter component.
Here was evidence that one star was moving around another. It was actually the first evidence that gravitational influences exist outside the solar system.
A binary star system in which both of the stars can be seen with a telescope is called a …………. …………
visual binary
A star like Mizar A, which appears as a single star when photographed or observed visually through the telescope, but which spectroscopy shows really to be a double star, is called a ……….. …………
spectroscopic binary
Edward C. Pickering (1846–1919), at Harvard, discovered a second class of binary stars in 1889—a class in which only one of the stars is actually seen directly. He was examining the spectrum of Mizar and found that the dark absorption lines in the brighter star’s spectrum were usually double. Not only were there two lines where astronomers normally saw only one, but the spacing of the lines was constantly changing. At times, the lines even became single. Pickering correctly deduced that the brighter component of Mizar, called Mizar A, is itself really two stars that revolve about each other in a period of 104 days.
Strictly speaking, it is not correct to describe the motion of a binary star system by saying that one star orbits the other. Gravity is a ……… ………..
mutual attraction
Each star (that orbit each other) exerts a gravitational force on the other, with the result that both stars orbit a point between them called the ……….. ……..
center of mass
We can estimate the masses of binary star systems using Newton’s reformulation of Kepler’s third law
where D is in astronomical units, P is measured in years, and M1 + M2 is the sum of the masses of the two stars in units of the Sun’s mass. This is a very useful formula for astronomers; it says that if we can observe the size of the orbit and the period of mutual revolution of the stars in a binary system, we can calculate the sum of their masses.
We measure the speeds of the stars from the Doppler effect. We then determine the period—how long the stars take to go through an orbital cycle—from the velocity curve. Knowing how fast the stars are moving and how long they take to go around tells us the …………… of the orbit and, hence, the separation of the stars in kilometers or astronomical units. From Kepler’s law, the period and the separation allow us to calculate the sum of the stars’ masses.
circumference
Of course, knowing the sum of the masses is not as useful as knowing the mass of each star separately. But the relative orbital speeds of the two stars can tell us how much of the total mass each star has. As we saw in our seesaw analogy, the more massive star is closer to the center of mass and therefore has a smaller orbit. Therefore, it moves more slowly to get around in the same time compared to the more distant, lower-mass star. If we sort out the speeds relative to each other, we can sort out the masses relative to each other. In practice, we also need to know how the binary system is oriented in the sky to our line of sight, but if we do, and the just-described steps are carried out carefully, the result is a calculation of the masses of each of the two stars in the system.
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