Sky Talk March 2010: Time Traveling Across the Sky

Stargazers are often referred to as “time-travelers.” This is because the more distant a celestial object the longer it takes for its light to reach us — ranging from minutes and hours in the case of viewing members of our Solar System to millions and even billions of years when it comes to looking at galaxies!

The March sky offers a great opportunity to “trip” through time (and space) using nothing more than the unaided eye and the Edmund Scientifics Star and Planet Locator as our guide. We’ll begin with some of the brighter stars, and then work our way outward to a couple of star clusters, a nebula and finally a galaxy. All are identified on the Locator itself, which should be set for around 8:00 p.m. local time. (A pair of binoculars or small telescope opens up a vast additional realm of targets, but for our purposes this month we’ll be concentrating on targets visible without optical aid.)

The brightest star in the entire sky — and one of the very closest — is blue-white Sirius in Canis Major. Its distance is 8.6 light-years, which means the light we see it by tonight left the star 8.6 years ago. Only a little further back in time at 11 light-years is crème-tinted Procyon in Canis Minor. Gazing at golden-yellow Capella in Auriga takes us back 42 years and rosy-topaz Aldebaran in Taurus 62 years. But for really big stellar time-jumps take a look at Orion’s two brightest stars — ruddy-orange Betelgeuse and blue-white Rigel. The light we see them by left these suns 520 and 770 years ago, respectively.

Viewing star clusters typically take us back thousands of years into the past, but there are three shown on the Locator that lie much closer in space and time. The Hyades are 130 light-years away and the famed Pleiades 375, both starry jewelboxes being located in Taurus, while the much dimmer Beehive Cluster in Cancer lies 590 light-years from us. To go further back in time, we must travel to the great Orion Nebula in Orion. It appears as a fuzzy-looking star in the sword hanging from the celestial hunter’s three belt stars. We are seeing light emitted by this stellar nursery 1,600 years ago and now just reaching us.

The biggest time-jump occurs when we turn to the galaxies, a handful of which are visible to the naked-eye. Brightest and most famous of these for Northern Hemisphere observers is the Andromeda Galaxy in the constellation of the same name. (But here just for the record, the grandest galaxy of them all is the one we live in — the magnificent Milky Way, shown on the Locator coursing diagonally across the March sky.) Best seen in the Fall and early Winter as an elongated pale glow, it’s located low above the northwestern horizon on early March evenings. When we view this remote star-city, we’re seeing the combined light of its some 500 billion suns which left there more than 2,400,000 years ago. Yes, stargazers truly are time-travelers as they look out at the wonders of the universe!

–James Mullaney
Former assistant editor at Sky & Telescope magazine and author of seven books on stargazing.