vColor, times of peak brightness clues to star distance
Q: We measure star distances up to about 100 light years using the parallax method. How
do we measure distances beyond 100 light years?
A: It's not 100 light years any more. Parallax is now
accurate to 500 light years away. Late in 1989, the
European Space Agency launched a satellite above Earth's
distorting atmosphere that offers a median error of a
thousandth of an arc second.
Beyond that, however, we must resort to other clever ideas.
One of the best relies on the color of stars.
STScI for NASA
First a word about star properties: they come in two types, A Cepheid variable star 56 million
light years away.
intrinsic and apparent (or extrinsic). Intrinsic properties
describe the actual star (independent of the observer), e.g.,
its radius, surface temperature, brightness, composition, mass. Apparent properties
describe how the star appears to us many light years away and depend, not only upon the
star, but also on external conditions, such as what's between us and the star and the
distance between us. A very-bright appearing star may simply be nearby. A star's real
brightness dims with distance.
We see three apparent star properties: position, brightness, and color. To understand a
star, we must figure out its intrinsic properties from what we see.
Stars come in a variety of types. For ordinary stars (i.e. the so called main sequence
stars, which includes our Sun and about 90 % of the known stars), a star's color tells us its
brightness because we can look up its brightness on a chart, called the HertzsprungRussell diagram. In 1911 and 1913, a couple of astronomers (E. Hertzsprung and H.N.
Russell) plotted a star's intrinsic brightness and its color for all known stars. The mainsequence star data falls on a diagonal line. Thus, we look up in the sky at a star and
determine its apparent color. Then we find its corresponding intrinsic brightness on the
H-R diagram. (See Further Surfing below for an example of a H-R diagram.)
Moreover, we can measure the star's apparent brightness by
looking at it. Now we're getting somewhere since we know
both its real and apparent brightness. Bingo! That gives us
the distance to the star since its intrinsic brightness
decreases as the square of the distance. Thus, we solve the
equation for distance.
apparent brightness = intrinsic brightness / (distance)²
Measuring distances to stars
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Up to 500 light years.
Up to 500 million light
years.
More than 500 million
light years.
This method of determining star distance from its color is called main-sequence fitting .
The main-sequence fitting technique enables us to measure distances to stars about
150,000 light years away, which takes us beyond the Milky Way
Our nearest neighbor galaxy, though, is 160,000 light years away. We need another
technique to measure distances to galaxies. Stars of varying brightness, called the
Cepheid Variables, enable us to measure beyond 150,000 light years.
Here's what the technique is based on: All stars, late in their life, change from mainsequence average-sized stars to swollen red giants. Most stars change again from swollen
red giants to pulsating variable stars before they finally die. These pulsating variable
stars, called Cepheids after the first one discovered, are very large, bright yellow stars.
They change in brightness regularly with periods of one to 70 days between peaks. This
lets us determine their brightness because the period is known to be directly related to the
brightness. Thus, a Cepheid with a 70-day period is intrinsically brighter than one with a
10-day period.
Here's how it works: We find a star whose brightness varies. We observe one peak
brightness and wait for the next peak brightness: that's its period. We look up the
corresponding intrinsic brightness of the star on the chart. Then, once more, knowing the
intrinsic and apparent brightness, we can determine the distance to the variable star. (See
Further Surfing for an example of the period-luminosity relation chart.)
Best of all, Cepheids are all over the place so they tell us how far away galaxies are up to
500 million light years away. Although, much beyond 20 million light years, only quasars
and clusters of galaxies put out enough light for us to detect.