Betelgeuse and its environs. Credit: Adam Block /Steward Observatory/University of Arizona

Tonight around 9:00 p.m. Eastern time, the Moon will occult (pass directly in front of) the planet Mars, and this event will be visible to esentially the entire United States! I’ve seen lots of occultations, and they’re really fun: you can see the Moon slowly creeping up on the planet and then taking a few seconds to cover it.

The timing on this event varies a lot depending on your location, so check this IOTA page to see the timing for you. It lists lots of major cities, so find the closest one and be out a bit earlier to make sure you don’t miss it! Binoculars help a lot here, Mars is amazingly bright right now and should hold up well even next to the full Moon. A telescope is best, of course, but you can just look up to see it too. I hope you have clear skies!

Note: My most recent article for Scientific American is a list of astronomical events I’m looking forward to in 2025, and is a highlight!

The Dark Energy Camera (or DECam) is mounted on a 4-meter telescope in Chile, and it’s a wee bit more sophisticated then your typical phonecam: for one thing, it’s a 570 megapixel camera. It has an amazing ability to get fine detail even over its huge 3 square degree field of view (for comparison, the full Moon occupies about 0.2 square degrees of sky). It’s designed to look at thousands of galaxies at a time and map their positions and colors, to help astronomers better understand the still-mysterious dark energy.

It also winds up taking devastatingly beautiful images in the process. Check out this incredible shot of the Antlia Cluster:

[Click to galactenate, and yes, you very much want to see the full 11,000 by 8,000 pixel image].

Antlia (the “Pump”, of all things) is a southern constellation and gives this cluster its name. But this isn’t a star cluster, it’s a galaxy cluster: It contains over 200 galaxies all held together by their mutual gravity.

As clusters go it’s not very big; many have thousands of galaxies in them. But Antlia is close to us at roughly 130 million light-years, the third closest such cluster (after Virgo and Fornax), making it ripe for study.

Also unlike most clusters it doesn’t have one singular massive central galaxy (like M87 is for Virgo), but does have two largish elliptical galaxies that dominate it (NGC 3268 in the center and NGC 3258 to the lower right). You can see hundreds more smaller galaxies around it.

There are also hundreds more in the background that look pretty similar (and zillions of stars in our own galaxy in the foreground, for that matter). Which brings up a good question: How can you tell which galaxies are in the cluster and which happen to be in the background?

A sure way is to get the redshift of each galaxy, measuring their velocity away from us as the universal expansion sweeps them along. More distant galaxies have a higher redshift, so it’s not hard to eliminate some galaxies that are much more distant.

But what if one is close to the same redshift as the cluster average, but not exact? How can you tell if it’s actually a member or not?

Veterinary costs up 60% over the past decade

Veterinarians nationwide reported that corporate managers pushed clinics to focus on profit, encouraging them to see more pets, order more tests, and upsell services.

Pet insurance could help you offset some of these rising costs, with some providing up to 90% reimbursement.

View Money’s top pet insurance picks to see plans starting at only $10/month.

SEE PLANS

That’s harder. There are some methods that work in statistical ways, like the nearest neighbor method, which looks at galaxies that are near each other and how likely they are to be associated with one another. That can be useful to get overall numbers but doesn’t do a great job at letting you point to a specific galaxy and say, “this one belongs”.

Another is to estimate the overall mass of the cluster, which tells you how much gravity it has. We can do that by looking at how much light all the galaxies emit, since that depends on the number of stars they have, or by taking spectra to see how rapidly the stars are moving in the individual galaxies; more massive galaxies have more gravity and the stars in them will move around more quickly.

Armed with the mass, you can look at the velocities of individual galaxies and determine if they’re moving slowly enough to stay bound to the cluster, or too quickly to be held on by it. That can help astronomers eliminate ones that happen to be passing by, or were ejected by the cluster after passing too closely to a more massive galaxy and suffered a slingshot effect. 

There are other methods, too, but none is perfect, though, so at some point we have to put an uncertainty on which galaxies are members and which aren’t. That’s a little bit frustrating but that’s life; not everything can be nailed down with precision, so you have to just deal with not knowing exactly. Hopefully, though, we can do a good enough job to get some of the scientific answers we want, and get estimates for other properties that, maybe, can be honed better sometime in the future.

It took me a long time to learn this lesson as I became a scientist: sometimes you just don’t know. It turns out to be an important one, one of the most important in studying science, actually. It’s best to know your limitations, of course; critical when writing a paper and you have to quote your uncertainties. But it turns out to be pretty helpful in life, too. Knowing when something is good enough will lead to a lot less frustration and save you time.

As long as you really do understand what “enough” really is. In astronomy as in life, that can be pretty tricky to figure out, too.

I could write more on this, but hey, I’ll take this lesson to heart and just end the article here.

Free subscribers may have noticed that I have started running occasional ads in the newsletter for various services. I’m doing this for the same reason anyone has done it since time immemorial: I’m trying to pay my mortgage. Just so’s you know, I won’t run just any ad; I check out the company first and make sure it’s legit and well thought of, because I’m thoughtful that way. Now, if you’re a paid subscriber and wondering what I’m talking about, that’s a perq of being a Premium subbie: the ads only show up to free subscribers. So if you prefer not to see them, then high thee over to the signup page and upgrade! I’m happy to welcome you either way. Thanks!

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!