This work was created by Dr Jamie Love and Creative Commons Licence licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

Multiple Star Systems, Novas and Supernovas Type I

by Dr Jamie Love Creative Commons Licence 1997 - 2011

If Algol's white star (Algol A) was a white dwarf (which it is not) we might expect this mass transfer to produce a nova someday.

Huh? What?

Ah, I thought that would get your attention. Allow me to explain this important process.

A binary system might contain a very massive star and a very small star. The massive star will be hotter and age faster than the smaller star but they will both be the same age. (Because they started from the same nebula. Right?) The faster aging star will be the first one to become a white dwarf (or neutron star or black hole). So multiple star systems can have a lot of variety within them.

This can lead to some very important and interesting astronomical events.

If a white dwarf is close enough to another star, the dwarf will use its powerful gravitational force to draw materials away from its neighbor. This is especially true if the white dwarf's neighbor is a red giant because the giant can't hold onto its huge ("fluffy") outer shell against the pulling force of the white dwarf's gravity. The white dwarf slowly strips the outer shell off its companion, pulling it along the orbital plane and creating an accretion disk around the white dwarf. [An accretion disk is also forming around Algol A but that is not my point here because Algol A is NOT a white dwarf.]

You will recall that a white dwarf is a "dead" star. It no longer has nuclear reactions to keep it "burning". Instead it radiates the latent heat it has accumulated during its past. However, the accretion disk delivers more material to the white dwarf and that material (most hydrogen) is heated up and put under pressure on the surface of the white dwarf. Eventually, in the lowest part of the accretion disk, at the surface of the white dwarf, nuclear reactions begin again. The white dwarf comes to life! But most of this energy is shielded - held at the surface of the white dwarf, by the incoming materials from the rest of the disk. Eventually the energy builds and an explosion occurs which blows away a large amount of the accretion disk. This material, surrounding the dwarf, is blown away at speeds up to 1500 kilometers per second. With the accretion disk temporarily removed and all that energy being released, the white dwarf now shines brightly as a nova.

Novas last only a few days. Eventually most of the energy has been released, so the white dwarf reverts back to its old, non-nuclear self, and a new accretion disk starts to form as the white dwarf gets back to its old trick of stealing materials from its companion's fluffy surface.

So, it might go nova again?

Yes, it might, but it takes some time for the mass to transfer to a new accretion disk and to recreate the conditions necessary to make the white dwarf explode again. However, several stars have gone nova several times in the past few centuries and we call those stars recurrent novas.

Where can I find them?

There are several to choose from but let me tell you about a particularly "famous" one. You may recall the tiny constellation CORONA BOREALIS (The Northern Crown) from our lessons in May. At that time I told you about R Coronae and how it sometimes dims - because of "soot" building up in its atmosphere. I also told you that there were some other amazing stars in this otherwise obscure constellation and now we return here for an example of the best-known recurrent nova.

Just outside the bowl, near Epsilon () Coronae lies the "Blaze Star" more properly named T Coronae. It usually has a magnitude of about 10 so you won't see it with your naked eye, or in my image here so I've put a circle where you would expect it. In 1866 it "blazed" to a magnitude of 2.2 (about as bright as Alphekka) but for only a few days. Then it settled down to its dim magnitude 10. However, in 1946 it blazed to a magnitude of 3. Of course, it stayed that way for only a few days and then returned to its lower magnitude.

I think it's worth keeping your eye on this part of the sky because T Coronae might blaze again. Wouldn't that be exciting! Unfortunately, if these recurrences happen on a regular time scale, we might not expect T Coronae to go nova until around 2026. We can only wait, watch and hope!

It just so happens that T Coronae's red giant, the star supplying the material in the mass transfer that drives this recurrent nova, also has mild fluctuations of its own brought about by its own complicated structure. But that's getting into even deeper astrophysics and I think we've covered enough.

In summary, a nova is a star that has brightened temporarily due to an explosion on its surface. The explosion is caused by the material from the accretion disk undergoing nuclear reactions at the star's surface, eventually leading to the rapid release of the stored up energy. The nova star is usually a white dwarf, but there is some evidence that a B or O-type star might be at the heart of some novas. The mechanism for those novas is probably a bit different from the novas caused by white dwarf "re-ignition". So, who knows, maybe Algol will go nova one day. (We can only wait, watch and hope! )

There are about 10 to 15 nova events each year in our galaxy and some of them might be recurrent nova. Amateur astronomers are often the first to discover novas so, as you gain experience and learn your astronomy, you might find one too.

For example ...?

On December 1st 1999 Alfredo Pereira, an amateur nova hunter in Cabo Da Roca (Portugal) discovered a 6th magnitude "star" where he knew there shouldn't be one! He was using 14 x 100 binoculars at the time. You don't need a telescope to discover these things! Just binoculars, an expert knowledge of the night sky and perseverance.
Alfredo noticed that this "star" was 2o NNW (north by northwest) of delta()Aquila, the 4th brightest star in the constellation of AQUILA the Eagle.

Alfredo reported this "new star" to the Central Bureau for Astronomical Telegrams and they confirmed that it was indeed a nova and gave it the name "Nova Aquilae 1999#2". [If Alfredo had discovered a comet it would be named after him but nova are just given numbers based upon when they were discovered. ] There had been only one other nova discovered in 1999 and that one was far too dim to be noticed by all but a few folks with really nice telescopes. Regardless, Nova Aquilae 1999#2 brightened to a magnitude of about 4, making it the brightest nova to be seen in the Northern Celestial Hemisphere for 25 years! Maybe it's a recurrent nova. We don't know. All we can do is watch and wait.

And hope! So how do I find it?

First find AQUILA by finding its brightest star Altair. You'll recall that Altair is part of the "Great Summer Triangle", a pseudoconstellation.

Now that you've found Altair you can complete the image of AQUILA. In this image I'm displaying all stars down to a magnitude of 6.5 (which is slightly dimmer than can be seen with the unaided eye). You may have to increase the brightness of your screen to see them all.

There is no particular way to draw the constellation but notice that I have connected delta()Aquila to zeta()Aquila and I connected delta()Aquila to gamma()Aquila. This forms a nice triangle.

Notice there is very little (actually nothing at this limit of magnitude) in the part of the triangle nearest to delta()Aquila and close to the line connecting delta()Aquila to zeta()Aquila.

On December 1st 1999 a new star appeared in the position in which I have drawn a circle. By December 2nd 1999 observers noticed that the nova had brightened to 5.4 and by December 3rd it was magnitude 4.0.

If you see a "star" north and slightly west of delta()Aquila you may have discovered that Nova Aquilae 1999#2 is a recurrent nova!
For comparison, delta()Aquila has a magnitude of 3.4 and I've added in the magnitudes of three dimmer but neighboring stars. You can use these dim "comparison stars" to help you keep track of the nova's brightness - assuming it returns. If you have a telescope with built in positioning, set the coordinates to RA 19h 23m 5.37s and declination of +4o 57' 20.1'' and have a look.

But don't knock yourself out. This is just an example. I don't know if Nova Aquilae 1999#2 is a recurrent nova and I don't even know (for sure) that it is part of a multiple star system. I just thought this would be a good time to tell you a story about how an amateur astronomer discovered a nova.

An important trait of a nova is that the white dwarf is not destroyed. Indeed it loses very little of its mass. In fact the white dwarf comes out of it a little bit more massive because it retains a small amount of the mass from the previous accretion disk. Once a star is reported as having gone nova, astronomers tend to keep an eye on it waiting for it to go "pop" again. However, if it's a Type I supernova there is no hope for a recurrence.

What's a Type I supernova? (We learned about Type II supernovas last month.)

A Type I supernova occurs when a white dwarf is pushed over its Chandrasekar's limit, which you learned about last month.
You will recall that a white dwarf is a planet-size chunk of carbon (and perhaps some other light elements) held up by electron degenerate pressure. Over time the white dwarf in a binary (or multiple) system may collect materials from its red giant neighbor and increase its mass. It may undergo several novas along the way but each time the white dwarf comes out of the explosion a little heavier than it was before. Eventually the white dwarf collects a mass approaching Chandrasekar's limit (1.44 solar masses).

You will also recall that as a star evolves, it becomes a red giant and usually explodes as a supernova Type II. (Very massive stars may end up as black holes and thus not explode.) If the core it leaves behind is less than Chandrasekar's limit the core will become a white dwarf. That's what will happen to our Sun in several billion years. A mass greater than Chandrasekar's limit cannot remain a white dwarf because its electron degeneracy pressure cannot stand up against the crushing gravity. Instead it evolves into a neutron star (held up by neutron pressure) or a black hole. That is the normal course of events of star evolution.

But when a white dwarf is prodded across Chandrasekar's limit by infalling materials - it blows up! Its carbon fuses and the entire white dwarf is blown to bits in a tremendous explosion that leaves nothing behind except a fast moving shell of debris. The entire star is destroyed and the energy given off in the explosion makes the dying star as bright as 400 billion Suns! That's more light than in all the stars in our galaxy combined. The material ejected from the explosion is a massive version of a planetary nebula, rich in elements and moving away from the center at incredible speeds. Eventually these materials will settle to make new nebula, stars, planets, etc.

I've always heard about supernovas but until now I never understood how they came about.

Now you know! Here's a review.

A Type II supernova is the result of the aging of a normal, average size, star. (Very big stars will not produce supernova Type II because they will become black holes.) These stars age into red giants - "fluffy" stars with a cool surface. The outermost layer, the envelope, still contains a lot of hydrogen because not all of it is burned up. When this star explodes, as a Type II supernova, its spectrum will show clear signs of hydrogen. One day our Sun will die as it explodes in a Type II supernova - giving off a hydrogen rich spectrum and leaving behind a white dwarf.
A Type I supernova is the result of a white dwarf being pushed over its Chandrasekar limit due to the infalling mass from a companion, so it can only happen in a multiple star system containing a white dwarf. There is less mass in the Solar System than Chandrasekar's limit so our Sun, will never end up as a Type I supernova. Because Type I supernovas are made from an exploding white dwarf, they have no hydrogen in their spectrums (they will have lost all their hydrogen when they evolved through the Type II supernova stage) or show only a trace of hydrogen collected from the accretion disk.
Astronomers carefully study the spectrum of supernovas looking for the telltale signs of hydrogen. If they find lots of hydrogen they know it was a Type II supernova and if they find no hydrogen (or very little) they classify it as a Type I supernova.

That should be enough to satisfy your curiosity but more recently a complication has developed. We now suspect that some Type I supernovas - the ones without hydrogen in their spectrum - are caused by the collapse of the cores of massive stars which have lost their hydrogen envelopes. These kinds of core collapses have a complicated physics like that of an exploding white dwarf so they should be classified as a Type II supernova. However, because their spectrum shows no hydrogen, they must be classified as Type I! We must stick with the classification based upon the spectrum. So, the Type II supernova is still called Type II because it has hydrogen in its spectrum. However, the Type I supernovas, without hydrogen in their spectrum, are now subdivided. Type Ia is the good old fashion "normal" Type I caused by a white dwarf being pushed over its Chandrasekar limit. Type I supernovas that are caused by the complicated core collapse of a massive star are classified as Type Ib and Type Ic (and I won't try to explain the differences between them! )

Well, I think the whole classification is stupid anyway.
You have to have a Type II supernova before a Type I supernova! Right?

Yes, you might think supernovas were numbered the wrong way around. When supernovas were first discovered and named we had no idea about star evolution or astrophysics. All we had to go on was magnitude. The names have to do with BRIGHTNESS.
Type I supernovas are brighter than Type II supernovas - and usually take longer to fade too.

Also note that each star can make only one supernova Type II and (if its neighbor supplies extra mass) only one Type I supernova. [If you don't understand why, think about how both these supernovas are made. Type II is from a star dying of old age and type I is from a dead star being blown to bits! You can only age once and you can only blow up once.] Therefore, supernovas are rare. On average there are about one or two supernovas (Type I or II) in a galaxy per century! Only 6 have been observed in the past 1000 years.
On the other hand, normal nova, like the Blaze Star (recurrent) nova, occur about once a month in an average size galaxy like our own and no star is destroyed in the process. It is because the star is not destroyed that it may rise again as a recurrent nova and show off again. (Although it never shows off as brightly as a supernova.) Therefore, novas are more common than supernova.

The Blaze Star is probably a recurrent nova and with each cycle it gets a little bit more massive. It's probably a white dwarf working its way towards the Chandrasekar limit. So one day, tomorrow or billions of years from now, it might come back BIG TIME! BOOM!!!
I wouldn't hold my breath waiting for Blaze Star to make a supernova (Type I) but it's a good excuse to learn your way around the night sky. One night you might see something that wasn't there before!

Of course, not all multiple star systems are going to go nova. However, multiple star systems are common and easy to observe. While Algol will not go supernova or even nova (probably), it's a fine example of a binary system undergoing mass transfer and its eclipsing behavior makes it well worth following. After all, you can't predict a nova but you can be certain of Algol's eclipse, so make a point to learn how to find it and watch it. Learn how to find PERSEUS and Algol.

What are those other stars near PERSEUS?

Ah, that's the constellation of ANDROMEDA and it's well worth learning your way around it. It's a large, complicated constellation with only three bright stars.

The one nearest PERSEUS is called Almaak. It's a binary (the primary is orange and the secondary is bluish) 120 light-years away and with an overall luminosity almost 100 times as great as the Sun. Don't bother trying to distinguish the Almaak binary unless you have a big telescope because they are separated by only 9.8 arc seconds (9.8''). Almaak has a magnitude of 2.1 so it is a perfect match to the "uneclipsed" Algol. Learn to recognize Algol and Almaak and you will have a way to tell if Algol is being eclipsed.
The next bright member of ANDROMEDA is Mirach, an orange star 88 light-years away and as bright as 115 Suns.
Alpheratz is the brightest star in ANDROMEDA and is only 72 light-years away.

These three second order magnitude stars, along with the two dim stars between them, seem to form a "warped CASSIOPEIA". I've always found this part of ANDROMEDA by first finding CASSIOPEIA and then imagining that I'm tugging CASSIOPEIA as I drift away from Polaris. This drops you right into ANDROMEDA's twisted W-shape.
Above (northward) of ANDROMEDA's "W" is the rest of this constellation. You should be able to make a fat ORION from this part of ANDROMEDA, using Mirach as one corner. The top of this hourglass-shape has a large extension that forms a "Y".

According to Greek legend Andromeda was chained to a tidal boulder (at the seaside) as a sacrifice to a sea monster. Perseus, Andromeda's loving husband, saved her by using some lateral thinking. You see, there was another monster called Medusa. She was a Gorgon, a hideous, snake-haired woman whose face turned people to stone if they saw it. Using the reflection in his shield to aim his sword, Perseus chopped the head off Medusa and carefully put it in a sack. (Without looking at it, of course.) Perseus then hurried to the seashore where his wife lay in chains about to be eaten by the sea monster. Perseus pulled the head of Medusa from the sack and showed it to the sea monster. The monster immediately turned to stone and everyone was happy (except the sea monster and Medusa. )

Perseus and Andromeda now live in the northern night sky. Some artists show him still clutching the head of Medusa with Algol as one of her eyes. Indeed, Algol's nickname is "the demon star".

I'll have a look at Algol (and hope I don't turn to stone). Are there other binaries around?

Yes, there are many beautiful binaries in the night sky but you need a telescope to see most of them.

Remember Alberio in the "beak" of CYGNUS the Swan? Well, it's actually a binary. The primary is yellow and has a magnitude of 3.1 while the secondary is blue and contributes its magnitude of 5.1 to the light we call "Alberio". They are separated by only 34'' (34 seconds of arc) so you will need a small telescope to resolve this colorful pair.

Castor (in GEMINI) is a binary. In fact, Castor's primary has two secondaries. A is the brightest of the three (of course) at a magnitude of 1.9. B has a magnitude of 2.9 and is due east of A but it is so close to the primary that it cannot be resolved without a good telescope. [Specifically, B is a mere 2.5 arc seconds from A with a position angle of 88o.] Together they give "Castor" a magnitude of 1.6. This pair has a revolution period of 420 years (but are not aligned for eclipsing, so don't try waiting around for it ). The third member of this multiple star system, C, has a magnitude of only 8.8 so you need magnification to see it. It's 1.2 arc minutes from the primary with a PA of 164o (so you know to point your binoculars about south-southeast of the primary).

Even familiar Sirius is a binary! However the primary (Sirius A) outshines its secondary (Sirius B) by ten thousand times and they are separated by only 4.5 arc seconds. Don't bother - even professional astronomers have trouble finding Sirius B in the glare of Sirius A.

Astronomers use a variety of imaging "tricks" to identify binaries and it is beyond the scope of this lesson to teach them to you. Even with a great telescope there are still some binaries that are difficult to resolve (distinguish).

Are there any "naked-eye" binaries (besides Mizar/Alcor)?

Yes. Plenty.

TAURUS has two binaries you should be able to see without aid. Stay up late enough on a nice July night you'll see TAURUS rise.

On a clear night look at the stars slightly below Aldebaran. The next bright star (theta or ) is really a pair of stars of almost identical magnitude. One is 3.4 and the other 3.8. [No, the pair is NOT the two that you see in this image. I'm talking about the upper of those two.] This pair has a PA of 346o, so you know that the slightly dimmer one will be slightly west of due north of the primary. [Right?] You might want to use binoculars in order to confirm that you have the right stars. They are about 5.5 arc minutes apart or half the separation of Mizar/Alcor so they are within naked-eye resolution. Barely.

Another great naked-eye binary is the "lone star" in between the two horns. (I've circled it here.) This pair is dimmer (4.7 and 5.1) but have a better separation (about 7 arc minutes) and a PA of 193o, so you will find the dimmer star slightly west of due south from the primary.

Oh, while we are in this neighborhood, I've got to point out Lambda Tauri() in the shoulder of the Bull. It's a binary system with one A-type star and one B-type star. This "star" is 326 light-years away and the secondary is only 14 million kilometers from the primary so you will never be able to resolve this pair. However, this is still a star worth watching because its secondary eclipses the primary every 3.953 days. It's an eclipsing binary like Algol! Most of the time Lambda Tauri shines with a magnitude of 3.4 but during eclipse, which lasts about 14 hours, its magnitude drops to 3.9. OK, this isn't as dramatic a change as Algol but it's still pretty good. Compare its brightness to the 3.6 magnitude of the star at the tip (nose) of TAURUS (Gamma Tauri or ) and maybe you'll notice a difference from time to time. When eclipsed, Lambda Tauri will be slightly dimmer than Gamma Tauri and when not eclipsed Lambda Tauri will be slightly brighter than Gamma Tauri.

That would be a difficult difference to notice.

Yes, it's difficult but try to make a comparison whenever you see TAURUS and I bet you'll learn to discriminate the maximum and minimum of Lambda Tauri by comparison with Gamma Tauri. Experienced amateur astronomers claim to be able to spot a 0.2 magnitude dip in Lambda Tauri caused by the eclipse of the secondary by the primary! Of course, some of these "amateurs" have some pretty sophisticated photoelectronic equipment to back them up!

Is there any mass transfer going on in Lambda Tauri?

I don't know (everything). But that reminds me of another interesting eclipsing binary and it's easy to find.

Remember LYRA in the Summer Triangle?
Its brightest star is Vega.
Well, the second brightest star in LYRA, Sheliak (also called Beta Lyrae with the symbol of ), is an eclipsing binary too! Its magnitude goes from 3.3, similar to Sulaphat (3.24 magnitude) down to 4.4, about the magnitude of those other two stars that complete LYRA's diamond, so it's easy to follow this variation by comparison to nearby stars.

This system's orbital period is 12.94 days but when the eclipse occurs it isn't very abrupt. Instead, this star's brightness changes in a smooth continuous cycle. This was quite a puzzle until someone figured out that the two stars are so close together that they actually distort each other's shape by each other's gravity! These two stars, which are far too close to resolve with any telescope, are not spheres. They are egg-shaped, with the tip of one "egg star" pointing to the tip of the other!

So, as you can see, binary (or multiple) star systems are full of interesting sites for all astronomers to enjoy - whether amateur or professional. They are the cause of supernovas Type I and (normal) novas. Those with a good amount of separation will not do nova "tricks" because they have no chance for mass transfer but they make for good observations because we can see both stars easily. On the other hand, binaries that are too close to resolve are often close enough to frequently eclipse each other (if the geometry is right) and they may undergo mass transfer. While we amateurs cannot see the actual "stars of the show" we can certainly see their effects as they go nova (perhaps regularly) supernova (only once) or eclipse (like clockwork).

When I see two stars that look close together, is it safe to say they are binaries?

NO!

Two stars may appear to be close together by chance. It's just a line of sight effect so you cannot trust it. As a matter of fact, with a little magnification you could really fool yourself into thinking that all the stars are part of a multiple star system!
It requires careful observations of the pair, including data about their movement and distances, in order to prove that two stars are truly a binary pair and not just lined up close to each other from our viewpoint. Always consulate a good star atlas if you want to know if a particular pair is really a binary. Astronomers spend a great deal of time cataloguing binaries so always consult the experts. [In November I'll tell you how astronomers determine star distances and calculate motion.]

Find some time to learn the constellations and stars in this lesson. This is a good excuse to stay up late one clear, summer night and really look at TAURUS like you never have before. By the time winter returns you will be ready to enjoy TAURUS all over again with a new knowledge of what an interesting constellation it really is. While you wait for TAURUS to rise you can enjoy the view of VEGA and become familiar with its eclipsing (and "egg-shaped") Sheliak binary.

Here's the northeast night sky again as seen around midnight on the 1st of July. As the month progresses you needn't stay up quit so late to see this view.

I've added the tiny constellation of TRIANGULUM to this image. (Cute, ain't it. )

Amateur astronomers take pride in having witnessed an Algol eclipse. Of all the eclipsing binaries Algol is the only one that you can actually see changing in front of your very eyes! (The others require that you remember the changes from night to night by making comparisons and taking notes.)

You know, I'd like to see an Algol eclipse but exactly when can I see it happen?

That requires a wee bit of work. You have to observe an Algol eclipse and then you can follow its cycles by tracking its period of 2.867315 days. What you need is that first eclipse to get you started. Below is a list of the times of Algol minima for the last part of the year 2000. Use it to prove to yourself that these minima occur every 2.867315 days. Then you can use the last day of the year 2000 to calculate more (beyond that date).

July 2000

3 at 8:03
6 at 4:52
9 at 1:41
11 at 22:29
14 at 19:18
17 at 16:07
29 at 12:55
23 at 9:44
26 at 6:32
29 at 3:21

August 2000

1 at 0:10
3 at 20:58
6 at 17:47
9 at 14:35
12 at 11:24
15 at 8:12
18 at 5:01
21 at 1:50
23 at 22:38
26 at 19:27
29 at 16:15

September 2000

1 at 13:04
4 at 9:52
7 at 6:41
10 at 3:29
13 at 0:18
15 at 21:07
18 at 17:55
21 at 14:44
24 at 11:33
27 at 8:21
30 at 5:1

October 2000

3 at 1:58
5 at 22:47
8 at 19:36
11 at 16:25
14 at 13:13
17 at 10:02
20 at 6:51
23 at 3:39
26 at 0:28
28 at 21:17
31 at 18:06

November 2000

3 at 14:55
6 at 11:44
9 at 8:32
12 at 5:21
15 at 2:10
17 at 22:59
20 at 19:48
23 at 16:37
26 at 13:26
29 at 10:15

December 2000

2 at 7:04
5 at 3:53
8 at 0:42
10 at 21:31
13 at 18:20
16 at 15:09
19 at 11:59
22 at 8:48
25 at 5:37
28 at 2:26
30 at 23:15

These are all in Universal Time (of course) so you will have to correct for your local time. These times represent the midpoint of Algol's minimum (magnitude) but remember that this minimum lasts about 20 minutes and the entire eclipse lasts about nine hours, so plan to spend plenty of time either side of the timepoints in order to see the whole event.

Next month a wonderful, annual event occurs in astronomy and the focus of that event is PERSEUS. Indeed, this event begins in late July so get out there and have a look at PERSEUS. You might see more than you expect. In the autumn I'll tell you about a wonderful sight in ANDROMEDA. So make a point to learn these two difficult yet important constellations and enjoy some late nights.

See you next month.
Wishing you "Clear Skies".
Jamie (Dr Love)




This work was created by Dr Jamie Love and Creative Commons Licence licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.