Premium subscribers dominate my sky

In BAN #1072 I wrote about a thermal infrared image of Earth and the moon taken by NASA’s ESCAPADE spacecraft. I noted that the press release said the temperatures seen ran from about -23 to 7°C, and I wrote, “that seems off to me”, since the equator should run warmer than that even at night. BAN reader Claude-Alain Roulet (who has a Physics PhD) pointed out to me that our atmosphere is opaque to thermal IR, so the emission we’re seeing is not from the ground, but from the atmosphere high above it, where temperatures are lower. That makes sense to me (and honestly I should’ve figured that out) so I retract my statement.

The annual Perseid meteor shower comes to a peak on the evening/morning of August 12/13 this year, and it’s always a good show. Depending on viewing conditions, up to 60 – 100 meteors per hour can be seen! For most folks it’ll be less, unless you can find a really dark site.

But it’ll be worth it; this year the moon is not up at all to interfere with its bright light — because on the 12^th there’s a total solar eclipse, which by definition means the moon is near the sun in the sky, so it sets at sunset and won’t be up until sunrise.

Your best bet is, again, find a dark site, but also one with wide open space so that nothing interferes with the horizon. The shower is best after local midnight (literally halfway between sunset and sunrise, so around 01:00 or so for the US), because that’s when the part of Earth you’re on is facing into the incoming meteoroids.

For more, you can read what I wrote in Scientific American for the 2023 shower, or Time and Date’s page, or for background info my Crash Astronomy episode on meteors.

There’s also this cool interactive animation from MeteorShowers.org of what the orbits of the meteoroids look like:

Good seeing and clear skies to you!

Dagnabbit, I wish astronomers could make up their minds.

Actually, that’s not true. Scientific arguments are fun, and show how we’re sometimes right at the edge of understanding.

In this case, the topic is a bit of a morbid one: When the sun dies, will Earth get eaten or not?

In about seven billion years, the sun will run out of hydrogen in its core that it can use to fuse into helium. This process releases vast amounts of energy, and is what powers the sun. The details are complicated, but when it runs out of hydrogen the outer layers of the sun will swell up and cool, turning it into a red giant. As time goes on it will shrink and expand several times, in fact.

When it’s a red giant the solar wind becomes much denser, and in a few hundred million years the sun will lose about half its mass. The strength of its gravity depends on its mass, so as the sun blows this wind the planets will be less strongly bound to it, which means they’ll slowly spiral away from the sun. For Mercury and Venus that won’t make much difference; the sun will expand enough to engulf them anyway.

But Earth is far enough away that it’s right on the edge. When the sun expands, Earth might be inside the sun…or it might not. It depends on a lot of factors, and whenever astronomers include a new one it seems the conclusions flips. At first it looked like we’re solar food, but then the next paper said nope, we’d be outside the sun, but then another bit of research says we escape that fate. It flip-flops a lot, and the last I heard we get et.

But a new paper just came out, and guess what it concludes? Guess! 

Yup. Maybe we’re safe. This time, they looked at tides [link to journal paper].

I wrote a lengthy description of tides back on the old Bad Astronomy site, and also for Scientific American. Oversimplifying a bit, when the sun becomes a red giant, Earth’s gravity will pull a bulge up in the sun’s material toward Earth. The sun will rotate very slowly by then, and Earth orbits around it much more rapidly. So in essence Earth will drag that bulge around with it, which steals energy from its orbital motion. That in turn will drop Earth closer to the sun. If it drops enough, poof! We get engulfed.

But does it? Will this effect counteract Earth moving away from the sun as our star loses mass and its gravity weakens?

What the astronomers found is that our fate depends on the details models of how the sun loses mass, and how the tidal evolution occurs. Using updated models (including mass loss rates from the star L2 Puppis, which used to be very much like the sun but is now a red giant), they find we just squeak by, and manage to stay outside the sun.

Nice! But not really! 

Why not? Because either way, we’ll be surfing through space just outside an enormous red-hot star. The swollen sun will be around a temperature of 4,000+°C, and will be so huge it’ll occupy half the sky! So even if we’re technically outside it, we’ll still be good and truly cooked. 

Of course, we’re talking about something that won’t happen for another six billion years or more, so it’s not exactly a pressing concern. On the other hand, studies like this really help us examine the details of how stars behave when they become red giants, and could help us better understand the behavior of other stars that are red giants and also have planets. We know that dying sun-like stars produce gorgeous and intricate planetary nebulae when they die, and the shapes of these objects can depend on whether the star consumed a planet (or if it has another star as a binary companion), so there’s a direct tie-in to this work. Also, it’s just a normal thing, I think, to wonder what our solar system might look like eons hence. 

We still don’t know, exactly, but we’re learning. Maybe, hopefully, one day we’ll find out what side of the sun we fall on.

You can email me at [email protected] (though replies can take a while), and all my social media outlets are gathered together at about.me. Also, if you don’t already, please subscribe to this newsletter! And feel free to tell a friend or nine, too. Thanks!