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Al Stahler: Looking up, looking back

Al Stahler

Tonight the moon is new – no moonlight to interfere with the light of the stars.

Once the sky grows fully dark (10 p.m. PDT), step outside and look up. Almost overhead, a faint band of light illumines the sky: the Milky Way.

A flock of birds … a herd of cows … a galaxy of stars.

Our sun is one of a hundred, maybe two hundred billion stars residing within our galaxy, named for the glow: the Milky Way Galaxy.

The Milky Way Galaxy is a spinning pinwheel of stars. The glow we see tonight comes from regions closer to the center, where stars are more jammed-together. The Milky Way glows brightest in Sagittarius, which will rise later tonight (earlier, in coming months). The Sagittarian glow marks the very center of our galaxy.

The Big Dipper is high in the sky tonight. Think of the Dipper’s bowl as a picture-frame; by the mid-twentieth century – before we had space telescopes – astronomers had found, just within that frame, tens of thousands of galaxies. Scattered in every direction, untold billions of galaxies drift through the universe.

Galaxies come in many shapes and sizes. We cannot see our own Milky Way Galaxy from the outside, but astronomers have mapped our galaxy from the inside. The maps reveal our galaxy to be of a type that is – without a trace of chauvinism – the most beautiful: The Milky way is a spiral galaxy.

Some fundamental questions: How do galaxies form? What creates those beautiful spiral arms?

Two objects, visible to the naked eye, yield clues.

The Big Dipper, high in the sky these nights, is a signpost, pointing to important parts of the sky.

In the bowl of the Dipper, the last two stars are, literally, “the pointers” – they point to Polaris, the north star (not a very bright star; it just happens to sit, 24/7, above Earth’s north pole.).

The handle of the Dipper also points, but not straight – the handle forms an arc. Follow the arc around, and the eye comes to (much brighter) Arcturus: “Follow the arc to Arcturus.”

Our galaxy is a ginormous pinwheel of stars. All the stars in our neighborhood move in roughly the same direction … but for Arcturus. If the stars around us were a flat target, Arcturus would be an arrow, flying – fast – through that target.

This fall, another object will be visible to the naked eye … an object not of our galaxy … at least, not yet. A faint fuzzball, Andromeda is another galaxy, outside our own … larger than our own … and heading, on a collision course, toward our own. Some billions of years from now, the Andromeda Galaxy and the Milky Way will collide. Like projectiles in a snowball fight, stars are unlikely to hit each other … but they will feel each others’ gravity … will pull on each other … eventually causing Andromeda and the Milky Way to merge.

Arcturus was likely once a member of another galaxy … one that collided, and merged with, the Milky Way. The star’s odd motion would then be a remnant of that collision.

An hypothesis (an educated guess): Galaxies form from much smaller clumps of stars, colliding and merging. Such mergers were once common.

To truly see how galaxies formed, we need to look back in time … way back in time … we need a time machine.

Looking back

Roughly twenty-five hundred years ago, on the eve of battle, the Greek army was out-numbered … but clever strategy led the Greeks to victory.

A courier was dispatched to bring the news to the distant capital. Running non-stop, the courier brought the news to Athens in just two days … whereupon he died from the exertion.

Footraces today memorialize that courier’s long-ago run, carrying the news of the battle of Marathon.

The courier’s run was impressive … though, by the time he reached Athens, the news was two days old. Today, we get news instantaneously, transmitted via radio waves. Like all electromagnetic waves (visible light, x-rays, infrared, ultraviolet and gamma radiation), radio waves travel at the speed of light … which can circle the Earth seven times in one second.

Electromagnetic radiation moves fast, but the distances between galaxies are great. The light we’ll see this fall left Andromeda two-and-a-half million years ago … we’ll see Andromeda as it was, two-and-a-half million years ago.

The information we get from galaxies is old news … but that’s perfect, if we want to look back in time. The farther out we look, the farther back we look, in time.

Of course, as things grow more distant, they also grow dimmer, which is why we need telescopes – light buckets – to collect and concentrate light coming from distant parts of the universe. Looking out … looking back … telescopes are time machines.

Large telescopes collect and concentrate light with mirrors. The larger the mirror, the more light the scope collects … and the farther back in time it can look.

The Hubble Space Telescope had a mirror eight feet in diameter. Our newest space telescope – the James Webb Space Telescope – carries a mirror twenty-one feet in diameter. Just beginning its mission, JWST will look far back in time … to show us, we hope, how galaxies first began to put themselves together.

In the sky

Friday night, rising in the northwest at 10:01 p.m., the International Space Station will, a few minutes later, fly directly overhead.

Al Stahler enjoys sharing science and nature with friends and neighbors, in The Union and on KVMR-FM. He teaches classes, for both kids and grown-ups, and may be reached at a.a.stahler11@gmail.com.

A spiral galaxy in The Bear.
Provided by NASA, JPL-Caltech, Galex Team, J. Huchra et al. (Harvard CfA)


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