Boötes tills the skies

Sunday, Jun 9, 2024 12:00 AM

Greetings, stargazers.

The very bright star near zenith at dusk is Arcturus. It is the fourth brightest star in the night sky, and a very tiny bit brighter than Vega, which can be seen rising over the eastern horizon at the same time. One way to remember the name and location of Arcturus, is that if you follow the curved handle of the Big Dipper, it makes an arc in the sky that “arcs to Arcturus.” Continuing that same line to the south, it “Speeds on to Spica,” the next bright star you encounter in that direction.

I always like to point out the color differences between Arcturus and Vega, because Arcturus is definitely more orange, and Vega is bluer, which means it is much hotter. Most stars are dim enough that our eyes only see a pale white dot, but these two stars are bright enough that we can detect their colors.

Arcturus is an orange giant star about the same mass as our sun. However, it has used up the hydrogen in its core, and is approaching the final stages of its lifetime. When there is no hydrogen left in the core to fuse into helium, a very nonintuitive thing happens. The core actually increases in temperature. Because there is no longer an outward pressure from fusion, gravity starts compressing the core. This compression heats up the core the same way compression in a diesel engine heats up the fuel-air mixture. The now-much-hotter core causes all the outer layers to expand and cool as their energy is radiated into space. As a result, Arcturus is a bit cooler than the sun, but since it is much larger it radiates vastly more energy.

Arcturus is the brightest star in the constellation Boötes. There is an umlaut that often gets overlooked in printed material, so instead of the more proper double articulation of the vowels, I hear it pronounced like the tiny shoes a baby would wear. Either way works for me to understand which constellation is being discussed. Boötes is one of the ancient constellations and is supposed to represent the plowman or ox driver. I think the shape is more like that of a kite, with Arcturus being at the pointy end.

Because Boötes is not near the Milky way, there are no open clusters or emission nebulae visible in that part of the sky. There are also no bright galaxies easily seen with small telescopes. However, the Great Wall, a grouping of galaxies in the direction of, but much bigger than Boötes is the largest structure in the observable universe.

This month:

Congratulations if you were one of the people who got to see the auroral display last month. It was cloudy at my house when the best displays were happening, but I heard that I could have stayed up later to get some holes through the clouds. I did get a bit of aurora with a camera the following night, but nothing that I could see with my naked eye.

The sun rotates at different rates depending on the solar latitude, but generally it takes about a month, and the grouping of sunspots that caused the big display last month reappeared. We could be in for more auroras, but predicting space weather is even more challenging than predicting terrestrial weather. Good luck with that.

I have heard discussions about the “great” planetary alignment early this month. It is still around, but since it is in the direction of the sun, an observer can only see it close to dawn. Also, several of the planets are about as far away from Earth as possible, so my thought is this is about the worst time to try to see them through a telescope.

The summer solstice (the shortest night) is on June 20, and the full moon is on the 22nd, so there might be a week in the middle of the month when it hardly gets dark at all.