Star Dust

by Darrin Gunkel

The Summer Triangle

You’re standing on the side of a mountain, about 7,000 feet above sea level. It’s a few minutes after sundown and the color filling the western sky has you absorbed. Until you turn to the east and notice something odd. The sky has a pinkish glow but for a dark band of blue along the horizon. This is the Earth’s shadow cast onto the upper reaches of our atmosphere. It’s visible for a brief time after sundown, while the geometry of our sun and planet are just right. Once night fully falls, rather looking at the shadow, you’re standing under it.

The pink glow is called the Belt of Venus, and when it appears, it’s time to start looking for the first stars and planets of the evening. Twilight’s a great time to find your way around the sky – it more closely resembles those constellation finder charts that tend to show only the brighter stars. Things can get confusing later on in full darkness, when the storm of summer stars can throw off even experienced stargazers.

This month, the show begins with the two brightest planets: Venus blazing 15 degrees (or three fist widths) above the western horizon, and Jupiter, 30 degrees up from due south. Both should be easy to spot by 9:30. Just north of east, Vega, the fifth brightest star in the sky (not including the sun) rides a little higher above the horizon than Jupiter.

Vega burns as brightly as it does for three reasons. First, it’s big: two and half times the size of our sun. Second, it’s hot: its surface registers 9500 Kelvin (the temperature scale astronomers use, based on absolute zero. Our sun’s surface is 5770 Kelvin. The average temperature of the Earth’s surface is 287 Kelvin, or 57.2 degrees Fahrenheit.) Vega’s hotter, larger, and brighter than the vast majority of the 200 billion to 400 billion stars in our galaxy. Finally, Vega’s nearby, a galactic neighbor at 25 light years.

Vega is also the anchor for the bright summer asterism, or pattern of stars, known as the Summer Triangle. The second star in the group, Altair, is rising due east after sundown. By 10:00, it should have cleared the murk of dust and haze near the horizon. Altair has an entourage. Just above and below are the slightly dimmer Tarazed and Alshain, respectively. Altair’s not as bright as Vega because it’s neither as big nor hot. In fact, it’s much closer, clocking in at 16.7 light years.

Neither of them, however, holds a candle to the final member of the Summer Triangle. Deneb, found about 30 degrees above north-northeast as twilight deepens into full night. It’s among the largest and brightest stars in the galaxy, a super-giant 100 million miles in diameter. That’s not a typo. Deneb is wider than the distance between the Earth and Sun. Intrinsically, Deneb is something like 55,000 times brighter than our home star. Move it to Vega’s distance and it would be clearly visible during the day and cast shadows at night. But it’s 60 times further away, shining at us across 1500 light years, so it only ranks as the 19th brightest night time star.

Incidentally, big, bright stars are rare. Our Sun is a good example, often misidentified as average, though anything but. It’s larger and brighter than 90 percent of the stars in our galactic neighborhood. Of our 50 nearest stellar neighbors, only seven are bright enough that we can see them without the help of binoculars or a telescope, and only three of those are truly bright, first magnitude stars. Relatively close neighbors Vega and Altair don’t even make that list. A few of the rest can be spotted with binoculars, but most are tiny red dwarfs, often closer in size to the giant planet Jupiter than to our sun, and invisible with anything other than a seriously large telescope.

The Great Rift

As the night deepens, dimmer stars fill up the sky: the little parallelogram that hangs like a pendant below Vega, marking the constellation Lyra; the splay of stars to the south of Altair, the constellation Aquila; the Northern Cross capped by Deneb. And then there’s the Milky Way, the collective glow of billions of stars too distant and dim to make out with eyes alone. Together their light forms what the !Kung people of the Kalahari call the Backbone of the Night. The Milky Way runs right through the middle of the Summer Triangle, and through the middle of it runs the Great Rift.

The Great Rift splits the Milky Way into two streams. The stars aren’t sparser here, they’re obscured by great clouds of cosmic dust: the star stuff that Joni Mitchell and Carl Sagan liked to point out we are all made from. And not just us. Star dust is everything in the solar system that isn’t hydrogen or helium (everything that isn’t the Sun, Jupiter, and Saturn, basically), every planet, asteroid, comet, meteor. Everything on or in every planet, asteroid, comet, meteor. The oceans, the continents, the volcano you’re camping on. Moreover, that star stuff fuels those volcanoes.

The earth is hot inside: cranking at 44 trillion watts. Half of that heat comes from radioactive decay – the breakdown over time of uranium, mostly, but also thorium, potassium and a few others, into lighter elements. This decay unleashes subatomic particles that crash into the other stuff the earth’s made of, and transfer their kinetic energy into that stuff, heating it up. This melts the Earth’s interior, creating the convection driving the plate tectonics fueling mountain – and volcano – building. (The rest of the heat is leftover from the Earth’s formation – also kinetic energy, but from numberless bits of cosmic dust in the Sun’s birth cloud colliding and coalescing under the influence of gravity.)

So where’d all that dusty stuff come from? Back to the stars – the big ones like our Sun, which end their lives as planetary nebulae: glowing shells of future star dust and gas that disperse into the cosmic wind. But to make the really heavy radioactive elements, like uranium, you need really big stars like Deneb. Starlight is (part of) the exhaust of nuclear fusion: hydrogen fusing to helium, and so on to heavier elements. To get the really exotic, unstable radioactive elements like uranium, you need the conditions found only in a supernova, the death-throe explosion of one of those super-rare giants. Super-rare, but remember, there may be a third of a trillion stars in our galaxy, and it’s been around for something like 15 billion years. Plenty of time for plenty of ancient Denebs to cough up enough heavy elements to keep planets like ours cooking up entertaining mountains.