17 (1) Flashcards
Perhaps the most important characteristic of a star is its ……………..—the total amount of energy at all ………….. that it emits per …………..
luminosity / wavelengths / second
For example, the luminosity of Sirius is about 25 times that of the Sun. We use the symbol ………….. to denote the Sun’s luminosity; hence, that of Sirius can be written as ………. ………….
25 LSun
To make the comparison among stars easy, astronomers express the luminosity of other stars in terms of the Sun’s luminosity.
We call the amount of a star’s energy that reaches a given area (say, one square meter) each second here on Earth its ……….. …………….
apparent brightness
Astronomers are careful to distinguish between the luminosity of the star (the total energy output) and the amount of energy that happens to reach our eyes or a telescope on Earth.
The process of measuring the apparent brightness of stars is called …………..
photometry
astronomical photometry began with ……………. Around ………. ………….,
Hipparchus / 150 B.C.E
he erected an observatory on the island of Rhodes in the Mediterranean. There he prepared a catalog of nearly 1000 stars that included not only their positions but also estimates of their apparent brightnesses.
Hipparchus made estimates with his eyes and sorted the stars into ………. brightness categories, each of which he called a ………………
six / magnitude
Measurements showed that we receive about ……….. times more light from a first-magnitude star than from a sixth-magnitude star. Based on this measurement, astronomers then defined an accurate magnitude system in which a difference of five magnitudes corresponds exactly to a brightness ratio of …………..
100 / 100:1
The color of a star therefore provides a measure of its intrinsic or true …………… temperature
surface
If we could somehow take a star, observe it, and then move it much farther away, its apparent brightness (magnitude) would change. But this change in brightness is the same for all wavelengths, and so its color would remain the same.
R 2
The hottest stars have temperatures of over …………… K, and the coolest stars have temperatures of about …………. K.
40,000 / 2000
In space, the Sun would look …………, shining with about equal amounts of reddish and bluish wavelengths of light.
white
The Sun looks somewhat yellow as seen from Earth’s surface because our planet’s …………… molecules scatter some of the shorter (i.e., blue) wavelengths out of the beams of sunlight that reach us, leaving more long wavelength light behind. This also explains why the sky is blue: the blue sky is sunlight scattered by Earth’s atmosphere.
nitrogen
One commonly used set of filters in astronomy measures stellar brightness at three wavelengths corresponding to ultraviolet, blue, and yellow light.
The filters are named: ………. (ultraviolet), …………. (blue), and ………. (visual, for yellow).
U / B / V
These filters transmit light near the wavelengths of
- ……….. nanometers (nm) for U,
- ……….. nm for B, and
- ………. nm for V.
- 360
- 420
- 540
The brightness measured through each filter is usually expressed in magnitudes. The difference between any two of these magnitudes—say, between the blue and the visual magnitudes (B–V)—is called a ……….. ……….
color index
By agreement among astronomers, the ultraviolet, blue, and visual magnitudes of the UBV system are adjusted to give a color index of ……….. to a star with a surface temperature of about 10,000 K, such as Vega.
0
The B–V color indexes of stars range from ……….. for the bluest stars, with temperatures of about 40,000 K, to ………….. for the reddest stars, with temperatures of about 2000 K.
−0.4 / +2.0
The B–V index for the Sun is about …………
Note that, by convention, the B–V index is always the “bluer” minus the “redder” color.
+0.65
Why use a color index if it ultimately implies temperature?
Because the brightness of a star through a filter is what astronomers actually measure, and we are always more comfortable when our statements have to do with measurable quantities.
