19 (3) Flashcards
The variability of Delta Cephei was discovered in 1784 by the young English astronomer ……….. …………
John Goodricke
This is just one more piece of evidence that stars really do evolve and change in fundamental ways as they age, and that being a cepheid variable represents a ……….. in the ………… of the star.
stage / life
Recent measurements indicate that the amount by which the brightness of Polaris changes is decreasing and that, sometime in the future, this star will no longer be a pulsating variable.
The importance of cepheid variables lies in the fact that their ……….. and ………… …………. turn out to be directly related.
periods / average luminosities
The …………. the period of the cepheid, the …………… the luminosity.
This ……….. ………….. was a remarkable discovery,
longer / greater
period-luminosity relation
The relation allows you to essentially “read off” how bright the star really is (how much energy it puts out). Astronomers can then compare this intrinsic brightness with the apparent brightness of the star. As we saw, the difference between the two allows them to calculate the …………..
distance
The relation between period and luminosity was discovered in 1908 by ………. …………
Henrietta Leavitt
Henrietta Leavitt was only able to show that luminosity was related to period. She could not determine exactly what the relationship is.
R 2
In the 1920s, ……… ……….. made one of the most significant astronomical discoveries of all time using cepheids, when he observed them in nearby galaxies and discovered the expansion of the universe.
Edwin Hubble
The periods of RR Lyrae stars are always less than ……… day, and their changes in brightness are typically less than about a factor of ……….
1 / two
Astronomers have observed that the RR Lyrae stars occurring in any particular cluster all have about the same apparent brightness. Since stars in a cluster are all at approximately the same distance, it follows that RR Lyrae variables must all have nearly the same intrinsic luminosity, which turns out to be about ……….. LSun.
50
As satisfying and productive as variable stars have been for distance measurement, these stars are …………. and are not found near all the objects to which we wish to measure distances.
rare
In this case, it turns out the H–R diagram can come to our rescue.
By looking at the spectrum of a start we can determine the temperature, pressure and whether it is a ………… star, a ……….., or a ………….
main-sequence / giant / supergiant
Suppose, to start with the simplest example, that the spectrum, color, and other properties of a distant G2 star match those of the Sun exactly. It is then reasonable to conclude that this distant star is likely to be a main-sequence star just like the Sun and to have the same luminosity as the Sun.
But if there are subtle differences between the solar spectrum and the spectrum of the distant star, then the distant star may be a giant or even a supergiant.
The most widely used system of star classification divides stars of a given spectral class into six categories called ………… ………..
luminosity classes.
luminosity classes. (6) are:
- Ia: Brightest supergiants
- Ib: Less luminous supergiants
- II: Bright giants
- III: Giants
- IV: Subgiants (intermediate between giants and main-sequence stars)
- V: Main-sequence stars
The full spectral specification of a star includes its luminosity class.
For example, a main-sequence star with spectral class F3 is written as F3 V. The specification for an M2 giant is M2 III.
once a star’s spectrum has helped us place it on the H-R diagram, As before, since we know how luminous the star really is and see how dim it looks, the difference allows us to calculate its …………
distance
(For historical reasons, astronomers sometimes call this method of distance determination ………… ……….., even though the method has nothing to do with parallax.)
spectroscopic parallax
The H–R diagram is anchored by measurements of …………
parallax
The distances measured using parallax are the gold standard for distances: they rely on no …………, only …………..
assumptions / geometry
Once astronomers take a spectrum of a nearby star for which we also know the parallax, we know the luminosity that corresponds to that spectral type. Nearby stars thus serve as benchmarks for more distant stars because we can assume that two stars with identical spectra have the same intrinsic luminosity.
R 2
Each technique described in this chapter builds on at least one other method, forming what many call the ………. ………. ………
- cosmic distance ladder*.
- Parallaxes are the foundation of all stellar distance estimates, spectroscopic methods use nearby stars to calibrate their H–R diagrams, and RR Lyrae and cepheid distance estimates are grounded in H–R diagram distance estimates (and even in a parallax measurement to a nearby cepheid, Delta Cephei).*
