19 (2) Flashcards
Parallax
As Earth travels from one side of its orbit to the other, it graciously provides us with a baseline of ……….., or about ……… ……… kilometers. Although this is a much bigger baseline than the diameter of Earth, the stars are so far away that the resulting parallax shift is still not visible to the naked eye—not even for the closest stars.
2 AU / 300 million
The first successful detections of stellar parallax were in the year …………..,
1838
Friedrich Bessel in Germany measured the parallax of …………….
61 Cygni,
Thomas Henderson, a Scottish astronomer working at the Cape of Good Hope measured the parallax of …………………
Alpha Centauri,
Friedrich Struve in Russia measured the parallax of …………………….
Vega
Even the closest star, Alpha Centauri, showed a total displacement of only about ………. ………….. during the course of a year.
1.5 arcseconds
Figure 19.6 shows how such measurements work. Seen from opposite sides of Earth’s orbit, a nearby star shifts position when compared to a pattern of more distant stars. Astronomers actually define parallax to be one-half the angle that a star shifts when seen from opposite sides of Earth’s orbit (the angle labeled P in Figure 19.6). The reason for this definition is just that they prefer to deal with a baseline of 1 AU instead of 2 AU.
With a baseline of one AU, how far away would a star have to be to have a parallax of 1 arcsecond?
The answer turns out to be 206,265 AU, or 3.26 light-years.
What is a Parsec (pc)
“the distance at which we have a parallax of one second (arcsecond).” and 1 AU as baseline
How far a way the Parsec in Kilometers?
This is equal to 3.1 × 1013 kilometers
(in other words, 31 trillion kilometers).
The distance (D) of a star in parsecs is just the reciprocal of its parallax (p) in arcseconds; that is,
Thus, a star with a parallax of 0.1 arcsecond would be found at a distance of 10 parsecs, and one with a parallax of 0.05 arcsecond would be 20 parsecs away.
Back in the days when most of our distances came from parallax measurements, a parsec was a useful unit of distance, but it is not as intuitive as the light-year. One advantage of the light-year as a unit is that it emphasizes the fact that, as we look out into space, we are also looking back into time.
The light that we see from a star 100 light-years away left that star 100 years ago. What we study is not the star as it is now, but rather as it was in the past. The light that reaches our telescopes today from distant galaxies left them before Earth even existed.
In this text, we will use ……………. as our unit of distance, but many astronomers still use parsecs when they write technical papers or talk with each other at meetings.
light-years
To convert between the two distance units, just bear in mind: 1 parsec = ……….. light-year, and 1 light-year = ……….. parsec.
3.26 / 0.31
The stellar neighbors nearest the Sun are three stars in the constellation of ………….
Centaurus
If Proxima Centauri is part of a triple star system with the binary Alpha Centauri, as seems likely, then its orbital period may be longer than ………….. years.
500,000
Proxima Centauri is an example of the most common type of star, and our most common type of stellar neighbor ……………………………… make up about 70% of all stars and dominate the census of stars within 10 parsecs (33 light-years) of the Sun.
Low-mass red M dwarfs
The measurements of stellar parallax were revolutionized by the launch of the spacecraft ………. in ……….., which measured distances for thousands of stars out to about 300 light-years with an accuracy of …………………………
Hipparcos / 1989
10 to 20%
In December 2013, the successor to Hipparcos, named ……………., was launched by the European Space Agency. Gaia is measuring the position and distances to almost one billion stars with an accuracy of a few millionths of an arcsecond.
Gaia
The breakthrough in measuring distances to remote parts of our Galaxy, and to other galaxies as well, came from the study of ………. ………..
variable stars
some stars are seen to vary in brightness and, for this reason, are called variable stars. Many such stars vary on a regular cycle, like the flashing bulbs that decorate stores and homes during the winter holidays.
R 2
Let’s define some tools to help us keep track of how a star varies. A graph that shows how the brightness of a variable star changes with time is called a ……….. ……….
light curve
The maximum is the point of the light curve where the star has its greatest brightness; the minimum is the point where it is faintest. If the light variations repeat themselves periodically, the interval between the two maxima is called the …………. of the star.
period
There are two special types of variable stars for which measurements of the light curve give us accurate distances. These are called …………. and ………… variables, both of which are ……………………………….
cepheid / RR Lyrae / pulsating variable stars
Cepheids are large, yellow, pulsating stars named for the first-known star of the group, ………. ………..
Delta Cephei
