When we look at distant stars and galaxies, we're looking at the past. That's because it takes a while for the light to travel to us, and as Albert Einstein has shown, the speed of light is as fast as anything can travel. Take, for example, Alpha Centauri, the closest star system to us; it still takes more than four years for us to see the light from those stars.
Smith's Cloud, a beautiful giant gas cloud moving at about 700,000 miles per hour, is located about 12,000 light-years from Earth, so when we're looking at it, we're seeing it as it was 12,000 years ago. Other stars and galaxies are tens or hundreds of millions of light-years away, so when we look at them, what we see is what was happening back when dinosaurs still roamed the Earth.
But astronomy isn't just about the past. Sometimes the future can be revealed too, in a manner of speaking. For example, we won't be around when the sun dies in about 5 billion years, but we've found a similar star to the sun that is reaching the end of its life, giving us the chance to witness what it will probably look like when our star reaches the end of the line.
The image at the top of this article was taken by the Hubble Space Telescope, and it shows Kohoutek 4-55. It's a planetary nebula named after Czech astronomer Luboš Kohoutek, who discovered it. Buried inside this cloud of gas is a dying star with roughly the same mass as our sun. It didn't always look like this, but as the star aged, the nuclear fusion reaction that powered it began to diminish, causing it to pulsate and cast off its outer layers, creating the gas clouds that make up the nebula.
What makes it so beautiful? As the dying star collapses under its own weight, the massive core produces ultraviolet radiation that ionizes the surrounding gas, making it glow at wavelengths within visible light. The Hubble image is a composite of three images, each taken at a specific wavelength to isolate the light coming from particular atoms of gas. The different wavelengths have been color-coded: Red is nitrogen gas, green is hydrogen and blue shows oxygen.
Something similar is expected to happen to the sun in about 5 billion years, until it cools further into what is known as a white dwarf, a dense stellar remnant that only produces a faint glow.
Not all planetary nebula look the same
This image is a composite of optical light and X-ray that shows the Cat's Eye Nebula. (Photo: NASA)
Take a look at another example. Above is a composite image of the Cat's Eye Nebula, a relatively bright planetary nebula in the constellation of Draco. It was discovered by astronomer William Herschel on Feb. 15, 1786. The Cat's Eye Nebula is a much more complex planetary nebula than Kohoutek 4-55. This dying star is casting off its layers in pulses. "Observations suggest the star ejected its mass in a series of pulses at 1,500-year intervals," writes NASA. "These convulsions created dust shells, each of which contain as much mass as all of the planets in our solar system combined (still only 1 percent of the sun's mass). These concentric shells make a layered, onion-skin structure around the dying star."
We won't be around to see this happen, but these distant, helpful neighbors are helping us understand our future.