The galaxy M100 is one of the
brightest members of the Coma-Virgo Cluster of galaxies of an estimated
2,500 galaxies, at 50 to 60 million light years away (see bottom text)). The
galaxy is in the spring constellation Coma Berenices and can be seen through
a moderate-sized amateur telescope. M100 is spiral shaped, like our Milky
Way, and tilted nearly face-on as seen from Earth. The galaxy has two
prominent arms of bright stars and several fainter arms. The galaxy does not
yield its spiral form easily. Faint dwarf galaxies are just to the north and
east of this coiled galaxy. To the north, NGC 4322 is a probable companion,
but NGC4328 to the east appears to be in the foreground.
This NASA Hubble Space Telescope image of a region of
the galaxy M100 shows a class of pulsating star called a Cepheid Variable.
Though rare, these stars are reliable distance indicators to galaxies. Based
on the Hubble observation, the distance to M100 has been measured accurately
as 56 million light-years (+/- 6 million light-years), making it the
farthest object where intergalactic distances have been determined
precisely. Hubble's high resolution pinpoints a Cepheid, which is located in
a starbirth region in one of the galaxy's spiral arms (bottom frame). The
top three frames were taken on (from left to right) May 9, May 4, May 31
1994, and they reveal that the star (in center of each box) changes
brightness. Cepheids go through these changes rhythmically over a few weeks.
The interval it takes for the Cepheid to complete one pulsation is a direct
indication of the stars' intrinsic brightness. This value can be used to
make a precise measurement of the galaxy's distance.
The precise measurement of this distance allows astronomers to calculate
that the universe is expanding at the rate of 80 km/sec per megaparsec
(+/- 17 km/sec). For example, a galaxy one million light-years away will
appear to be moving away from us at approximately 60,000 miles per hour. If
it is twice that distance, it will be seen to be moving at twice the speed,
and so on. This rate of expansion is the Hubble Constant.
These HST results are a critical step in converging on the true value of the
Hubble Constant, first developed by the American astronomer Edwin
Hubble in 1929. Hubble found that the farther a galaxy is, the faster it
is receding away from us. This "uniform expansion" effect is strong evidence
the universe began in an event called the "Big Bang" and that it has been
expanding ever since.
To calculate accurately the Hubble Constant, astronomers must have two key
numbers: the recession velocities of galaxies and their distances as
estimated by one or more cosmic "mileposts", such as
Cepheids. The age of the universe can be estimated from the value of the
Hubble Constant, but it is only as reliable as the accuracy of the
distance measurements.
The Hubble constant is only one of several key numbers needed to estimate
the universe's age. For example, the age also depends on the average density
of matter in the universe, though to a lesser extent.