Staring Down the Horns of the Bull

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Mar 16, 2016
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Our brains are not terribly good at determining distances.

A pair of eyes affords us binocular vision, and that gives us depth perception out to a few meters. But beyond that we have to rely on other cues, for example relative size.

In astronomy it’s even harder. Objects in the sky tend to be pretty far away. But over the years we’ve gotten pretty good at it, and we can measure the distances to stars surprisingly well.

The photo above, taken by master astrophotographer Rogelio Bernal Andreo, shows a pretty nice-size chunk of the sky. It’s about 5° across (10 times wider than the full Moon) and shows the lower part of Taurus, the bull. This constellation has an almost iconic V shape, seen as the horns of the bull. This photo covers the lower spur of the V.

A lot of the stars in the shot are at about the same distance from us. That’s because the horns of the bull are also mostly stars that are physically associated with each other, part of a cluster called the Hyades. These stars were all born around a half billion years ago or so, and lie at a distance of 150 light-years from Earth.

That’s pretty close as galactic objects go—our galaxy is 100,000 light-years across! So this cluster is our close neighbor. Incidentally, astronomers recently found a Neptune-size planet orbiting a low mass star in the Hyades, so that's cool too.

However, not all the stars you see in that photo are actually members of the cluster. The brightest star in the picture is Aldebaran, and it is one of the few loners; it’s only 65 or so light-years from us, much closer than the Hyades! That’s one reason it looks so much brighter.

Another is that it’s an intrinsically luminous star. It’s a red giant, a star that used to be much like the Sun long ago, but ran out of hydrogen fuel in its core. When a star does that it swells up, cools off, turns red/orange, and becomes very bright. Aldebaran is nearly 50 times the width of the Sun, and more than 400 times brighter. Even from 650 trillion kilometers away it’s one of the brightest stars in the sky.

Now look at the photo again, and ignore the stars in it. See that faint, fuzzy, brownish stuff strewn throughout? That’s real. It’s background dust—small grains of silicates or complex carbon molecules created when stars are born and when they die. It litters the galaxy like smog on a hot day.

The brown color is curious to me; you don’t see that much in celestial objects. I found an article by astronomer R. Jay GaBany saying the dust reflects blue light from stars but also reddens light from stars behind it; together that gives it the mauve coloring. That’s an interesting idea. I’m not sure that’s all that’s going on here, but it seems like a good working hypothesis.

Much of this dust is located very far above the plane of the galaxy, probably blown there by the winds of dying stars—a poetic thought! In the case of the material in Andreo’s photo, we’re looking at Taurus at a low angle above the galactic plane, so there’s lots of dust in that direction (just look at nearby Orion, not far from Taurus, which is positively radiant in infrared, where dust glows).

Another thing: Because Taurus is a zodiac constellation, the Moon sometimes moves through it and also occasionally passes directly in front of Aldebaran, blocking it out. That’s called an occultation. I caught one in January 2016, and it was fun to watch (I even caught it on video). Aldebaran occultations by the Moon happen several times in 2016, so check to see if you’re in a place that’ll get one.

It’s funny to think: The Moon is the closest astronomical object in the sky. Aldebaran is 1.6 trillion times farther away, the Hyades twice as far as that, and the dust ranging from about that distance to hundreds of times further yet.

And even that is nothing compared to even the nearest galaxies. It’s staggering.

Yes, our sense of distance is lousy, but we have these big brains to make up for it, allowing us to figure out ways to cast our rulers. But that doesn’t mean our brains can’t be overwhelmed by the numbers we get. It’s a lovely irony, isn’t it?