If you like to watch sunsets, you’ll have a reason to stick around a little longer for an encore during the next couple of months.
When the sunlight begins to fade, we’ll see Venus and Mars together about a fist’s width above the horizon. Venus will keep getting closer and closer to Mars until they finally meet on July 13. All the while, they’ll be racing toward Regulus, the brightest star in Leo.
Who will get to Regulus first? Mars has the lead now, but dusk after dusk, speedy Venus will keep closing the gap. And just when you think you’ve got this contest figured out, there’s a surprise ending….
Let’s look to the West tonight, June 18, to see where things stand. Wherever you are, wait about half an hour after sunset, keep watching the area where the Sun went down, and you’ll see this:
Venus, of course, is very bright (magnitude -3.9), and Mars is easily visible, too (magnitude 1.8), even with some light pollution.
Regulus (magnitude 1.4) is actually a teeny bit brighter than Mars, and in practical terms, you can find it at the bottom of a group of stars (an asterism) within Leo that looks like a sickle, or backwards question mark:
Why do Venus and Mars appear close to each other in the sky right now? Because the Solar System on June 18 looks like this:
As the Earth spins counterclockwise, we see the Sun go down, and then about an hour and a half later, we see Venus and Mars set as well.
If we let our simulated planets keep orbiting for awhile, by July 13 we have this:
So on July 13, Venus and Mars will practically meet in the sky!
When they do meet, they’ll still be short of Regulus, so that means Venus will win the race, right? Well…
The thin crescent Moon springs up out of nowhere and sashays to an easy win! Show-off.
Venus and Mars will be very close together from July 11-15 and should make for especially lovely viewing not long after sunset. But you can start watching for them right now, any evening you can, and follow them as they get closer and closer to each other.
Now, I don’t mean to pick on our beloved Moon, of course. On July 11, see if you can catch it as a very thin crescent. Just after New Moon, young Luna is never too far from the Sun. It hangs near the horizon after sunset and makes a barely discernible arc:
Just one day later, on July 12, the Moon will be up off the horizon a little higher, and we’ll get a group photo not unlike this one from 2015:
Venus finally makes it to Regulus on July 21:
And Mars does the same on July 29:
So we have a month and a half of easy but compelling viewing ahead of us!
I can see why Venus and Mars are so interested in Regulus. It turns out to be a fascinating star, one that is spinning so fast (surface speed 700,000 miles per hour) that it bulges out at the center and looks like an oval. In fact, if Regulus were to spin just 10 or 15% faster, it would fly apart.
The reason for Regulus’ fast spin seems to be that another star orbits it. That star used to be a red giant, but Regulus sucked in so much of its material that all that’s left is its core, a white dwarf. If you think of Regulus as a ball hanging from a string, you can get that ball to spin pretty fast if you spray it with a hose along its edge. The hose water is the material Regulus sucked in, and that’s probably how it got spinning so fast.
Regulus is only 77 light-years away, and in 200 million years or so, it could go supernova. Here’s how that could happen:
In 100 million years or so, all the hydrogen at Regulus’ core will have fused into helium, and Regulus will turn into a red giant. When Regulus gets big enough, it’ll share its outer layer (mostly hydrogen) with the little white dwarf that orbits it. The core of Regulus will get closer and closer to the white dwarf, and that whole outer hydrogen layer will be ejected. Then we’ll have two helium cores that draw closer and closer together. When Regulus’ core is done fusing helium into carbon and oxygen, it’ll collapse further. Eventually it’ll start sucking in material from the white dwarf, and that additional gravitational pressure will push on the helium at Regulus’ core enough to make it suddenly start fusing again, and that’s your re-ignition. BAM-O! This would be called a “Type dot-Ia” supernova, which is like a mini-version of a “Type Ia” supernova.
A cute little event, but close enough to Earth that we’d surely be well aware of it.
So that’s why Venus and Mars are so interested in Regulus!
Now, Venus can pass directly in front of Regulus (an “occultation”), but that is really rare. In fact, from 1000 to 3000 CE, one of our planets occults a first-magnitude star only 11 times:
So Venus will indeed pass right in front of Regulus, but you’ll have to wait 23 more years for that. I’ll be 75 then, but if I’m still posting stuff on Daily Kos, I’ll be sure to let you know. 😁