Telescope, Lord Rosse and the spiral galaxies


By Tom Burns - Stargazing



More than a few years ago, I built a telescope that I could use to see the spiral structure of galaxies. It costs about $1,000 to build such an instrument, but I counted myself among the fortunate to be able to do it so cheaply. A year passed as I waited impatiently for the 17.5-inch-diameter mirror to arrive from its manufacturer. I pounded the telescope together out of cheap plywood and scrapped cardboard tubing in one frenzied weekend. Then I slapped on a coat of leftover house paint.

I was not, am not, and will never be an expert craftsman. The thing looked terrible.

But it worked just fine, thank you very much. During my long wait, I had built, badly, more than two-dozen smaller telescopes. They resolved the cloud bands and the Great Red Spot on Jupiter. They made globular star clusters explode into thousands of fully resolved stars.

But that was not nearly enough. My secret desire, my overriding passion, was to see the pinwheel-like structure in spiral galaxies. I wanted to tread the path that William Parsons, third Earl of Rosse, had trod 150 years before.

Galaxies may be made up of hundreds of billions of stars, but they are exceedingly far away. To see their structure requires a telescope of large light-gathering power, which in turn requires a telescope mirror with a large diameter ground and polished to a paraboloidal shape within an accuracy of .0000002 inches across its entire surface. Thus, virtually the entire cost of my telescope went into a commercially made mirror. Thus, I was willing to wait a year to get it.

The same telescope mirror would have cost a king’s ransom just 100 years ago. Big telescopes weren’t available by mail order. The avocational pursuit of stargazing was only for the very rich, who had the cash and the free time to spend on the universe.

Such was the case with the third Earl of Rosse. Born in Ireland in 1800, he served in the British Parliament from 1823-1834. He did so more out of a sense of lordly duty than anything else.

What is a rich member of the ruling class to do when he is able to retire from public life at the tender age of 34?

Lord Rosse decided to pursue astronomy, and he had the resources to do it in a big way. On the grounds of Birr Castle in the center of Ireland, he built the largest telescope in the world.

He had little help in doing so. He trained the workers on his estate to be telescope technicians. In those days, telescope mirrors were cast from metal, so Rosse constructed an enormous forge in which he created a mirror blank with the unheard-of diameter of 72 inches.

Once the blank was cast, Rosse had to develop new methods for grinding and polishing it to the correct shape, a daunting task that was finally completed after many months of strenuous labor.

But that effort was nothing compared to the task of mounting the mirror in a telescope. He set its tube between two enormous stone walls. In that configuration, he could move the telescope only up and down and not side to side. Luckily, the stars move, rising and setting like the sun and the moon. So Rosse waited until interesting parts of the universe appeared directly in line with the mirror. He was, if nothing else, a patient man.

The telescope, called the Leviathan of Parsonstown, was finally ready in 1845. Almost immediately, Rosse began to observe the “starry nebulae,” mysterious, faint patches of light. No one had been able to figure out exactly what they were.

Rosse’s telescope revealed what no other telescope on the planet had been capable of seeing. The faint fuzzies resolved into spirals of light that looked for all the world like children’s pinwheels.

Rosse’s efforts were, of course, very important to our understanding of the larger universe, but they were just as important to our understanding of our own cosmic neighborhood, the Milky Way.

It is, of course, your galaxy, and you’d think we’d know a lot about it already, and we do. The trouble is that we are in it. Its very proximity, size, and density make it difficult to estimate its structure.

Simple naked-eye observation reveals that it is disk shaped and that we are near the outside of the disk. That’s what makes it look like a streak of light across the sky.

However, astronomers actually found out much more about the overall structure and functioning of our galaxy by looking at other galaxies than by observing our own.

As Lord Rosse did, you can make those observations yourself in a typical backyard telescope. An example is M51 in Ursa Major, the Big Bear. The galaxy is a classic spiral, a child’s pinwheel of a hundred billion stars. Rosse drew the spiral structure of M51, sometimes with startling accuracy and sometimes, well, not so much. You’ll find a reproduction of one of them here: https://amazing-space.stsci.edu/resources/explorations/groundup/lesson/basics/g44/.

Of course, the big question when you see typical characteristics in other galaxies is whether they are really present in your own. That seems to be the case, often in surprising ways. A larger backyard telescope reveals a smaller smudge near M51. It appears to be about the same distance as the larger galaxy. An even larger telescope reveals a bridge of light between the two. Rosse lovingly drew that bridge. M51’s more powerful gravity is apparently sucking the stars right out of the satellite galaxy.

In fact, such galactic mergers seem fairly common as galaxies like M51 and our own careen through space, acquire satellites, and then sweep them up.

Given the large distances between galaxies these days, such galactic interactions seem unlikely. However, one implication of the Big Bang theory is that galaxies were a lot closer to each other 10 billion years ago. Big galaxies were bound to collect smaller brethren. Gravity has kept them together all this time.

Our own satellite galaxies, the Magellanic Clouds, are twisted and distorted — bent out of shape — by the intense gravitational power of the Milky Way.

But did our own Milky Way absorb other galaxies? The answer turns out to be a big yes!

The most stunning evidence that our own galaxy is the product of such galactic mergers is right there in front of your nose.

One of the reasons our galaxy is shaped like a spiral is that its stars are rotating around the center of the galaxy. Our own star, the sun makes a spin in 100 million years or so. But the orbits of some stars, including the bright star Arcturus in the constellation Bootes, are weird – tilted or elongated or both. Some astronomers speculate that their odd orbits are a result of their foreignness. They may have been absorbed from other galaxies a very long time ago.

In fact, a galactic merger features in our future in a big way. Watch the skies, my friends! Keep looking at M31, a naked-eye galaxy in the constellation Andromeda. It will continue to get bigger because it is screaming closer to us. In a scant four billion years, the 2.5 million light years between us will evaporate and the two galaxies will merge for a time.

Because of the enormous distances between the stars, it is unlikely that any of the stars of the Milky Way will bump into any of the stars of M31. However, their clouds of unused hydrogen will compress our clouds of unused hydrogen. The resulting rounds of star births will light up both galaxies for a long time after they pass completely through each other. Afterward, the two galaxies will go on their merry ways, somewhat the better for the experience.

We don’t have that problem with another of our galactic neighbors. M33, sometimes called the Pinwheel Galaxy, in the constellation Triangulum (the Triangle) is a bit farther away at 2.6 million light-years.

Because we see it face-on, its beautiful spiral shape shows up in medium-sized amateur telescopes. Look for a faint, fat S shape. Although it’s not visible to the unaided eye, binoculars will show it as a small fuzzy patch to the west of the most acute tip of the triangle.

Here you will find Rosse’s drawing of M33: seds.org/Pics/More/m33rosse.jpg>.

M33 is considerably smaller than its larger cousins, the Andromeda Galaxy and the Milky Way. At only 50,000 or so light-years wide, it contains only enough stellar material to make up about 8 billion suns. (Compare that with the 300 billion or more stars that make up our own Milky Way.)

Like all spiral galaxies, M33 is slowly turning. Astronomers estimate that stars out on its edge take up to 200 million years to make one turn around the galactic periphery.

Rosse had made a discovery of great importance, but he was left with a deeper mystery than the one he started with. The objects went from unidentifiable fuzzy patches to unidentifiable spirals. He still didn’t know what they were or why they were spirals, and nobody else could figure it out either.

The answer wouldn’t come until more than half a century after Rosse’s death in 1867. Using even bigger telescopes and advanced research techniques, astronomers finally determined that the pinwheels were island universes, galaxies of hundreds of billions of stars separated from each other by unimaginable distances.

But there I was, with a telescope I had slapped together in my garage, tracing out the spiral arms that Rosse himself had seen for the first time over 150 years ago in a telescope the size of my house. And I was one up on even the venerable Earl. I knew what they were.

It took me a couple of years to save the $1,000 it took to buy that telescope mirror. I endured a full year of waiting while my mirror was prepared. At the time, the three years seemed a lifetime. A lifetime later, the wait seems like nothing at all.

We have our troubles these days, no doubt about it. But measured against Rosse’s time on Earth, it seems glorious indeed.

Fellow and sister stargazers, count your blessings. We live in a wonderful age that allows even schlubs like you and me to see and understand the glories of the universe.

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By Tom Burns

Stargazing

Tom Burns is the former director of the Perkins Observatory in Delaware.

Tom Burns is the former director of the Perkins Observatory in Delaware.