Al Stahler: looking up, looking back


Tonight the moon is new – no moonlight to interfere with starlight.

Once the sky turns completely dark (10:00 PM PDT), get out and look up. Almost above our heads, a faint band of light illuminates the sky: the Milky Way.

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

Our sun is one of one hundred, perhaps two hundred billion stars residing in our galaxy, named after its 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 the stars are closer together. The Milky Way shines brightest in Sagittarius, which will rise later tonight (earlier, in the coming months). The Sagittarius 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; in the middle of the 20th century – before we had space telescopes – astronomers had found, just inside this frame, tens of thousands of galaxies. Scattered in all directions, countless billions of galaxies drift across the universe.

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

Some fundamental questions: How are galaxies formed? What creates those beautiful spiral arms?

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

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

In the Dipper Bowl, 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 hours a day). 24, 7 days a week, above the North Pole of the Earth. ).

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 huge windmill 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 that is not from our galaxy…at least, not yet. A faint ball of fur, Andromeda is another galaxy, outside of ours… larger than ours… and heading, on a collision course, towards ours. In a few billion years, the Andromeda galaxy and the Milky Way will collide. Like projectiles in a snowball fight, the stars are unlikely to collide…but they will feel each other’s gravity…pull into each other…eventually causing Andromeda and the Milky Way.

Arcturus was probably once a member of another galaxy… the one that collided and merged with the Milky Way. The strange movement of the star would then be a vestige of this collision.

A hypothesis (an educated guess): galaxies form from much smaller clusters of stars, colliding and merging. Such mergers were once common.

To really see how galaxies formed, we have to go back in time…far back in time…we need a time machine.

Looking back

About two thousand five hundred years ago, on the eve of battle, the Greek army was outnumbered… 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 of the effort.

Today’s foot races commemorate the long run of this courier, heralding the Battle of Marathon.

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

Electromagnetic radiation travels fast, but the distances between galaxies are large. The light that we will see this fall left Andromeda two and a half million years ago…we will see Andromeda as it was two and a half million years ago.

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

Of course, as things get further away, they also get darker, which is why we need telescopes – buckets of light – to collect and focus light 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 bezel collects… and the further back in time it can go.

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 travel far back in time…to hopefully show us how galaxies began to form.

In the sky

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

Al Stahler enjoys sharing science and nature with his friends and neighbors, in The Union and on KVMR-FM. He teaches classes, for children and adults, and can be contacted at [email protected].

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


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