How Far Can We See? (part one)

Astronomy enthusiast Matt, whose favorite celestial object is the three-star asterism (recognizable star pattern) Orion’s Belt, dropped quite a few questions into the Ask An Astronomer box. We’ll start with this one, since it’s perfectly timed for autumn stargazing:

“Is there a specific star that represents the farthest a person can see with the naked eye?”

I love this question, because it has two answers. First, the most distant object that a person with average eyesight can see with the naked eye is…
...not a star! It’s a galaxy, specifically, the Andromeda Galaxy.

Originally called the Great Andromeda Nebula, it became known as the Andromeda Galaxy after the famous astronomer Edwin Hubble confirmed its galactic nature in the 1920s. At an estimated 2.5 million light years away, it’s the nearest spiral galaxy to our Milky Way. The Andromeda Galaxy is more or less comparable to our home galaxy in size and mass, but astronomers believe it has a significantly higher star count.

Keep in mind that one light year is nearly six trillion miles, so the distance to the Andromeda Galaxy is pretty hard for us to wrap our heads around. But amazingly, with a little bit of effort, we can see it! Let’s gaze.

1) You’ll need a dark site, away from city lights, to see this “faint fuzzy.” Before beginning your hunt, be sure to dark adapt to maximize the acuity of your night vision. To dark adapt, simply avoid all white light for 20 minutes before stargazing. Then use a red flashlight during your stargazing session, to maintain your dark adaptation.

2) About one hour after sunset, face east. Look well above the eastern horizon for a large, four-star square. Each side of the square is about two fists wide, if you hold your fist at arm’s length against the sky and measure across the knuckles. This is the asterism called the Great Square of Pegasus.

Star maps created with Your Sky

3) Next, find the Chains of Andromeda, the two strands of stars that arc to the left (north) of the star that marks the Square’s lower left corner. Now look above the middle star of the upper chain for a faint, fuzzy patch. A Persian astronomer of the 10th century called it the “little cloud,” an apt description. If you spot it, you’re gazing upon a collection of about one trillion gravitationally-bound stars, what the farsighted philosopher Immanuel Kant called an island universe.

If you can’t see it, you may need to try again at a darker site with less light pollution. Additionally, your sky transparency, or atmospheric clarity, may be negatively impacted by the presence of clouds, haze, dust, or humidity. If this is the case, try again when conditions are improved.

When you look up at the night sky, every star you see is in the Milky Way. The exciting thing about spotting the Andromeda Galaxy—in addition to it being the farthest object you can see naked eye—is that you’re seeing a world beyond your galactic neighborhood. That’s right, it ain’t local.

For the second answer to Matt’s fine question, tune in next week. And until then, happy hunting.

The Way

There are an estimated 100 billion galaxies in the universe, and we occupy just one of them, the Milky Way.

It was the great 20th century American astronomer Edwin Hubble who determined that many mysterious telescopic objects thought to be emerging solar systems were in fact galaxies— immense star systems lying beyond the Milky Way. The Milky Way was not, it seemed, the whole enchilada. This discovery expanded our cosmic horizons and gave us our first inkling of just how immense the universe was.






Since Hubble’s landmark discovery, we’ve learned quite a bit about our cosmic 'hood. We now know that the Milky Way is a barred spiral type of galaxy. A spiral galaxy is shaped somewhat like a pinwheel, with curved arms radiating out from a center that is densely packed with stars. In a barred spiral, the center has an elongated shape.

We also know that there’s a supermassive black hole lurking at our galaxy's center. A black hole is an object so dense that nothing— not even light— can escape from its gravitational field.

Courtesy NASA/JPL-Caltech

In addition, we’ve learned that the Milky Way contains around 400 billion stars. We know it’s around 200,000 light years in diameter; one light year is the distance light travels in one Earth year, nearly six trillion miles. Knowing these numbers doesn’t necessarily mean we mere mortals can fully grasp the size and distance involved. The implication for a short-lived species such as Homo sapiens is that space travel to other parts of our galaxy would require thousands of successive generations of people on each pioneering space ship.

However, like all the generations of our species that came before us, we can see the Milky Way on most any night. If we look skyward from a dark site away from urban light pollution, we can enjoy an edge-on view of the star-packed arms of our platter-shaped galaxy. Because the Milky Way is a horde of stars too numerous and faint to be resolved (separated into distinct points of light) with the naked eye, we see it as a hazy band of light arching across the sky. Some folks seeing the Milky Way for the first time mistake it for a long, lingering cloud.

The poetic name for our home galaxy comes to us from the Latin Via Lactea (Milky Way), which in turn derived from the Greek word for milk. A number of other cultures also saw the band of light as a stream of milk. But around the world, there have been numerous names given to this distinctive celestial object. Here are just a few:

- Silver River
- Winter Street
- Path of White Ashes
- Birds’ Way
- Straw Road
- Pilgrims’ Road
- Great Serpent
- River of Heaven
- Heavenly Girdle
- Road of Souls
and my personal favorite, from the Polynesians:
- Long Blue Cloud-Eating Shark

My pet name for the Milky Way is simply “the Way,” because it represents for me a way of being and a way of seeing. I learned an important lesson a number of years ago on a camping and observing trip in southern Arizona. I had spent the better part of a night glued to the eyepiece of my telescope, hunting various galaxies, globular clusters, and nebulas on an observing list and methodically ticking off those “faint fuzzies” as I found them. Straightening up from the eyepiece to stretch my back, I faced south and audibly gasped. The woods south of the clearing where I and my partner were observing were on fire!

After a few heart-pounding seconds, I noticed that the raging firestorm licking the sky above the trees was black-and-white. Black-and-white flames? Then it dawned on me that what I thought was a forest fire was the Summer Milky Way, which had risen in all its magnificence while I was attached like a barnacle to my telescope.

Because I am fortunate to live and observe in New Mexico, which has excellent observing conditions, I thought I had seen the Milky Way. But I had never seen it like I saw it that night. The combination of superb transparency (atmospheric clarity) and a deep, dark, black-as-the-ace-of-spades sky background made every tendril and wisp of the Way pop out in blazing, 3D relief.

I almost missed the finest view of the night, a deeply satisfying naked-eye spectacle that flickered across my retinas and seared itself into my memory. And here’s the strange lesson: sometimes even serious sky observers have to remember to just look up.

Mother and Child

we remember Stephanie Keese, 1980-2008


Some deep-sky objects never lose their allure, even for veteran observers who have gazed at them hundreds or thousands of times through binoculars or telescopes. One of these is the magnificent Whirlpool Galaxy, 31 million light years away.

This spectacle of the northern sky is a showstopper at public star parties and observatory events. No one can forget his or her first telescopic glimpse of two galaxies interacting, one large and one small. It’s an iconic pairing I like to call “Mother and Child.”

Although I don’t normally venture outside of our own Milky Way galaxy in this blog, I will occasionally make an exception. This is one of those times.




The Whirlpool Galaxy is also known as M51, shorthand for Messier 51. This is its designation in the deep-sky catalog of the famed 18th century French comet-hunter Charles Messier (pronounced MESS ee yay). Technically, M51 refers to the large spiral galaxy only; the small, irregular companion galaxy has its own catalog number, NGC 5195. But in practice, most of us refer to these interacting galaxies collectively as M51, since they’re something of a package deal.

Messier himself discovered the ‘mother,’ and the ‘child’ was discovered by an astronomer pal of his. However, because of the limited resolving power of their telescopes, they thought the objects were nebulae (pronounced NEBB-you-lee; plural of nebula), clouds of gas and dust.

The 19th century English astronomer Lord Rosse discovered M51’s spiral nature through his whopping, six-foot-diameter “Leviathan” telescope in Ireland. It was the first time spiral structure had been recognized in a celestial object. But it was the great 20th century American astronomer Edwin Hubble who ultimately determined that many of the objects called “nebulae” (including M51) which were thought to be emerging solar systems were in fact galaxies--immense star systems lying beyond the Milky Way.

Restored Leviathan at Birr Castle, Ireland
Courtesy of Birr Scientific and Heritage Foundation

A dark site away from urban and suburban lighting is required to see the galactic twofer in all its glory. This is an ideal time of year to view this object, as it’s high in the sky. You can spot the Whirlpool Galaxy using binoculars, but you’ll need a telescope to see detail. Its proximity to the Big Dipper makes it fairly easy to locate.

1) You’ll need to face north, so if you don’t know the cardinal directions at your location and you don’t have a compass, make note of where the sun sets on the horizon. That spot is approximately west. Stand with your left shoulder to the west, and you’ll be facing approximately north.

2) Wait at least one hour after sunset to begin observing, so that twilight’s over and your sky’s good and dark. On Friday, May 23, you’ll have a couple hours of dark before the Moon rises. For the rest of the observing week, the Moon will rise even later each subsequent night.

3) Facing north, locate the Big Dipper, the distinctive seven-star asterism (recognizable star pattern) that looks like a giant, long-handled saucepan. It will be due north and somewhere between the zenith, the point directly above your head, and the northern horizon. The saucepan will be oriented upside-down. Point your binoculars or telescope at the point marked on the star map below, just off Alkaid (pronounced AL-kayd), the final star in the saucepan’s handle.

Chart created with Your Sky

Although just off the handle of the Big Dipper, which is part of the constellation Ursa Major the Great Bear, the Whirlpool Galaxy is actually in the lesser-known constellation of Canes Venatici the Hunting Dogs (pronounced KAY-neez vee-NATT-uh-sigh).

4) In binoculars and with good sky conditions, you should see an elongated fuzzy blob with two bright cores. A telescope will help you separate the blob into two distinct objects. In an eight-inch-diameter or larger reflector, you should clearly see spiral structure in the large galaxy. A large-aperture telescope and good sky conditions are needed to see “the Bridge,” the arm of the spiral galaxy that appears to reach out and touch the little galaxy.

In reality, the little galaxy is passing behind the large galaxy and has been doing so for hundreds of millions of years. But the gravitational pull of this galactic glide-by has disturbed and excited gas clouds in the spiral galaxy’s arms, setting off maelstroms of star formation. In the Hubble Space Telescope image below--the most detailed ever taken of M51--the new stars can be seen as bright blue clusters sprinkled along the sweeping arms.

It seems the child in this family portrait is quite precocious.