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.
R 2
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.
R 2
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.
R 2
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
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
