Night Sky for Week of July 20, 2026

[00:00:00]Greetings everyone, this is Jeff Wilkerson Professor of Physics at Luther College bringing you what to look for in the night sky. We're talking about the week of July 20th this time around. So, on that night, the 20th, in that evening into the morning of the 21st, the moon is just short of 45% full. So, we got a little bit of moon interference this week, and it sits just below Spica, uh the bright star in Virgo. Uh four nights later, the 24th into the morning of the 25th, it has filled out to be almost 80 85% full, and it sits below the bright red star Antares in Scorpius.

[00:00:34]Uh tracking across this direction, by the 26th, the last evening of the week into the morning of the 27th of the week we're talking about here, it's 95% full and in Sagittarius. Now, if you were watching this last week, and I don't know why you wouldn't have been, uh we started talking about objects, deep sky objects in Sagittarius, and we broke Sagittarius into lower, middle, and upper uh regions, south, um middle, and and north regions of Sagittarius. We looked at the middle region. We could this week go on and talk about one of the other regions, but because the moon is bright and uh moving toward Sagittarius, we're going to talk about parts that places that are east of there. So, a little bit farther away from that moon uh that's impinging on there. Uh but, before we get there, we want to talk about Venus, and I see I forgot to write anything on the board relative to Venus. I try to do this in advance cuz if I write while I'm talking, those of you who go back uh what, 5 years, whenever we started doing these things a long time ago, you know that I just started writing on the board as I was talking, and boy, could nobody read that. Um but, Venus in the evening sky, we've been talking about Venus a few weeks ago, we talked about how Venus is coming toward us right now, about as rapidly as it comes toward us. So, it's changing size size relatively rapidly, but it's only changing size at the rate of about 1 arc second per week. Uh so, it starts this week at about 19 arc seconds, and it ends the week at about 20 arc seconds. And the end of next week it's going to be about 21 arc seconds.

[00:02:03]Now, for those of you who maybe you recall this, we've talked about this some. You've got finger width at arms length is 1 degree, roughly speaking.

[00:02:11]1/60 of that is an arc minute. 1/60 of an arc minute is an arc second. So, 1 arc second is 1/3600 of a degree. And so, this you know, this but this is big for us. Now, the issue that we have here, if you want to try to measure the the size of Venus changing, now is your best chance, a great time to do it. But what I recommend doing is taking a picture of Venus every night that you can over the next month. And as you do that, what's going to happen is I would make a graph. I would plot the time down here, the date, and I would plot the the measured size up here. Because we talk about astrophysical seeing, we talk astronomical seeing. We say we the atmosphere is turbulent and it buffets light around and it causes objects to appear bigger than they are. So, stars should be pinpoints of light, but we don't see them as pinpoints of light because of this this scintillation is the word that astronomers use for it.

[00:03:09]Great story there talking about scintillation and the way physicists use that word, but that's a story for another time. Is to say we we often for most of us the seeing, we talk about astronomical seeing and that's how much a pinpoint of starlight gets spread out into a diameter of light and that diameter's often two or three and maybe even as bad as four if things are really turbulent and bad arc seconds. So, that means the difference between this and this can more can be swallowed up some by the turbulence of the atmosphere. One way to overcome that is to take photographs many many many nights, make this graph, and what you would see is you would see it grow like that, but it's it's it's going to be noisy around there. So, any two points, you might look at that that and say, "Well, I didn't change at all." But, it really did. You just needed to see that upward trend in all of that noise around there. This is the power of taking lots of data and looking at graphs. You know, when I wrote a book a couple of years ago, one of the it was a weird little book about nothing and and and all kinds of different things.

[00:04:11]But, the the one of the things one of the ideas in there is how powerful graphical analysis like this can be.

[00:04:19]So, this is one of just the little things that that that's part of who I am. It's part of how I think about the world and how I interact with the world.

[00:04:26]And so, it shows goes popping up all over the place. And I recommend doing that if you want a project over the next month, try to imagine the size of Venus in a photograph and and and see if you can and see that size grow that way.

[00:04:37]Now, what we're going to look at is we're going to look at Capricornus over to Aquarius. We're going to start with Deneb Algedi. We've looked at Deneb Algedi in this series of short videos a lot. Markab is the bottom right star in in the Great Square of Pegasus. It won't be up until after midnight now, probably. And so, but the moon's washing out the evening sky. So, we want a late evening into the morning observation anyway. We'll we'll we'll make a triangle with Altair. And Deneb Algedi is the faintest of those three stars. Remember, Altair is the southernmost star of the the summer triangle.

[00:05:14]We like that a lot, too, um to think about the summer triangle. It's an important group of an important asterism for us. But, Deneb Algedi has 2.9 magnitude stars. Fainter, it's not a lot fainter than Markab. It is quite a bit fainter than Altair. And it's got Nashira is that little point of a star right right by it. That means tail of the goat. So, this is the end of of Capricorn that we see there. Now, from Deneb Algedi over to Skat, which is the delta star in Aquarius, is about 16 and a quarter degrees. A fist at arm's length is about 10 degrees, so a little over one and a half fist widths at arms length from Deneb Algedi over to Scat.

[00:05:53]Now, what we're we're going to do with that, it's a it's it's a slightly fainter star, but still about about like Deneb Algedi.

[00:06:00]We're going to make a parallelogram. And it's not quite a parallelogram, but it looks like that. You drop south and west 7 and a quarter degrees from Deneb Algedi and you've got the beautiful globular star cluster M30. Uh fit binoculars find it your small telescope.

[00:06:16]It's a good globular star cluster. We talk a lot about globular star clusters this time of year. It's a great example.

[00:06:20]You drop 7 and 3/4 degrees down from Scat to NGC 7293. That's the Helix Nebula. This is a planetary nebula. This is a low mass star like the sun that has ejected its outer envelope. And you see that outer envelope of gas stretching into space. The Helix Nebula looks kind of like it has two overlapping lobes that are expanding out into space. As it expands it gets larger and easier to find. A lot of these look like stars.

[00:06:44]They're so small. This does not look like a star. This is a big object. You don't need a lot of these you need a lot some magnification to be able to tell that they're not stars. This you do not, but it's faint. As it spreads out the contrast of the the brightness variation with the background, the contrast with the background gets less and less and less as the gas gets thinner and thinner. And this is this is a doesn't have a is not a high contrast object, but it's a good object.

[00:07:09]Any relatively small telescope you should be able to see either one of these. This one in binoculars. That one might be hard in binoculars.

[00:07:15]It might but I'm thinking you know a 4-in telescope is going to do the job here without a doubt. Now, so a couple of one thing of interest. I find this fun.

[00:07:27]Maybe you don't find this fun.

[00:07:29]But about a half a degree to the west of M30 the globular star cluster here below Deneb Algedi is 41 Capricorni. So in some systems are how we identify stars we just give them numbers. And so so you just start at one and you go through there and you get to star 41 and 41 Capricorni is a binary star with a very faint 5.2 magnitude. So it's it's technically bright enough if you got dark skies, you could see it with your naked eye. Uh but binoculars would would help. Um then you want your telescope because just about uh you know, let's say you have a 10-in telescope, you might be able to see this 11.5 magnitude companion. It's got a faint companion, that's what makes it hard. 5 and 1/2 arc seconds is a nice separation of those two stars for you to be able to see the two stars, but 11 and 1/2 is a faint faint star. Uh and so that's it's it's fun. Give that a try and see if you can split that binary star. Well, the reason that makes it fun for us right now is we go over to the Helix Nebula and 4° west of the Helix Nebula is 41 Aquarii, the same number, but instead of 41 Capricorni, we're at 41 Aquarii. Just random. Just random luck that that number of star uh is uh quite random, but that that that number of star is right there. But it is just just sort of random that that's also a binary star with two components separated by about five arc seconds. And this is a 5.5 and a 6.7. This has a nice color contrast. You're going to enjoy this binary star more than this one, I think, uh due to that color difference and the fact that that 6.7 magnitude star is easy to see just about any telescope. Any telescope you got is going to find this as a good binary star. So give that binary star a try.

[00:09:03]Now for your real challenge objects of the week, you've got a great globular star cluster, you've got a great planetary nebula, you've got a couple of nice uh binary stars, one that's a lot easier than the other one. Your real challenge objects are going to go 3° north of the Helix Nebula and 4° south. You drop 4° south of the Helix Nebula and you've got a galaxy pair of NGC 7284 and 7285.

[00:09:24]So 7284 and 7285, they are a pair of galaxies that are merging. They're interacting and you see a stream of gas that that's that's been stripped out of those galaxies. Two spiral galaxies, uh not easy, uh but if you've got a 10-in or 12-in scope, you should be able to see this. If you got a 6-in scope and you're taking photographs, this would be a great thing to photograph, a great region of the sky. Go 3° north and you got another pair of galaxies. NGC 7301 is is harder, trickier than either one of these. So, again, you got a 10 or 12-in scope, you can see it. It's not going to be quite as easy, not going to be quite as impressive. It's also got a very faint companion, and that faint companion is PGC 88 1157.

[00:10:03]And so, this is a This this one you're This one I don't think you're going to see it unless you've got to take a photograph of the region.

[00:10:10]It's also very close there. These galaxies, I don't believe are interacting. Get PGC 88 1157 is more distant. But, it's little slashy galaxies and there's spiral galaxies that we've got in here. Good examples of these things if you want an observing challenge. That's your observing challenge this week. Your great objects are the M30 and the Helix Nebula, two really fine binary star systems, and then you got Venus. And you can start tracking the size of Venus. Oh, it drops. We didn't say that. It's It's down to 60% full. And it's going to drop It's getting close to half full up here.

[00:10:44]And so, you can watch it also become get close to a crescent phase over the next few weeks. And then you got the moon moving from Virgo to Scorpius to Sagittarius. So, as always, thanks for watching everybody, and we hope you have a great week ahead.