On the human time scale, the Milky Way galaxy is eternal. The starry Milky Way shined down on the first humans and will continue to shine on Earth for billions of years more. Eventually, though, the stars of the Milky Way will burn out. The galaxy will remain, but it will no longer look like a bright pinwheel. In fact, it won't look like much of anything at all -- and neither will the other galaxies sprinkled through the universe.
These galaxies move through space as a single unit, bound together by their mutual gravitational pull.
The number of galaxies in the Local Group is uncertain because astronomers keep finding new residents of this galactic neighborhood. Most of the newer members are small and faint, and some are hidden behind the glowing veil of the Milky Way.
In summer, in a clear place with dark night skies, an irregular glowing band arcs high overhead. The ancients likened it to a stripe of milk spilled across the sky.
Observations by Hubble Space Telescope and ground-based instruments show that the first galaxies took shape as little as one billion years after the Big Bang, which probably took place about 13 billion to 14 billion years ago.
There are two leading theories to explain how the first galaxies formed. The truth may involve a bit of both ideas.
One says that galaxies were born when vast clouds of gas and dust collapsed under their own gravitational pull, allowing stars to form.
The most beautiful galaxies are called spirals. The Milky Way is a spiral, and so is the Andromeda galaxy (M31).
Astronomer Edwin Hubble was studying an object in the autumn constellation Andromeda known as the Andromeda Nebula. Viewed through a telescope, it looks like a pinwheel, with bright streamers wrapping around a big bulge in the middle. At the time, most astronomers thought the Andromeda Nebula and similar objects were bright pockets of matter inside the Milky Way.
The most successful search method used to date is called the transit method. If a planet passes directly between a star and the observer, it blocks out a tiny portion of the star's light. Using this technique, the Kepler space telescope has discovered thousands of confirmed or suspected exoplanets, while ground-based transit searches have found hundreds more.
The discovery of many giant planets orbiting other stars shows us that planets -- at least Jupiter-like planets -- probably are common. There is little chance of life on such planets, though. To find life, astronomers are trying to detect small, rocky planets similar to Earth. This lies beyond current abilities, although it may not be far off.
This question has captured the imaginations of scientists, theologians, and philosophers for millennia. And while we don't know the answer yet, we do know that our solar system is not unique.
No one can say for sure what the largest or brightest star is. With 100 billion galaxies in the universe, and billions of stars per galaxy, there are just too many stars to measure. Here are a few realtively nearby stars that astronomers have been studying.
