Where can I find a white dwarf? Or a planetary nebula?
You need a good telescope to find them. Remember, a white dwarf
is as dim as a red dwarf, like Wolf 339. That's why they
are both low down on the H-R Diagram. The closest white dwarf
I know is called Sirius B. It's only 8.6 light-years
away and orbits the brightest star in our night sky, Sirius. This white dwarf is smaller than Neptune but as massive as the Sun. Sirius B has a magnitude of 8.5 so it is
too dim to be seen. Remember, with the naked eye you can only
see objects as dim as magnitude 6. Binoculars would allow you
to see objects as dim as Sirius B but Sirius(A),
with its magnitude of -1.5, drowns out the glow of its dwarf companion.
So don't run out to buy a telescope to find it.
A white dwarf that still has its planetary nebula is a rare sight because the nebula disappears in about 100,000 years. That's a very short period of time compared to the age of stars. You have to be lucky to have one in your neighborhood. Timing is everything. Fortunately we are lucky enough to have such a blessed event nearby - the Ring Nebula.
This photo of the Ring Nebula was taken through a very good
telescope and shows all the features you have just learned. [Credit: The Electronic Universe Project, Nelson Caldwell]
The ring is actually a sphere of material ejected from the star. Its ring appearance has to do with its geometry.
The Ring nebula has a magnitude of 9.7 so it's too dim for most binoculars but within the range of a small telescope. If you have such a telescope you might want to try to find the Ring Nebula but be warned it won't be as pretty as this picture. | ![]() |
Before you can find the Ring Nebula you must first be able to identify the constellation of LYRA, the Harp and the first step to identifying that constellation is to find its brightest star, Vega. There are two ways to find Vega and they are both worth knowing.
Using the northern sky, draw a line from Capella (in AURIGA)
towards Polaris, but slightly towards the Dippers. Then
continue over to the other side of the sky. The first bright star
you come to is Vega.
This is a nice way of finding Vega because it reminds you that Capella and Vega are on opposite sides of Polaris, but sometimes you can't see Capella (because of clouds or maybe it's below the horizon) so you should also know how to find Vega by the southern route, using Regulus. Imagine a line from Regulus (in LEO) to the last star in the Big Dipper's handle (Alkaid), then continue it on for a similar distance and you will find Vega. | ![]() |
Vega is very bright because it's only 26 light-years away and over 50 times more luminous as the Sun. This gives it an apparent magnitude of 0.03 in our sky. Vega's brightness along with its white color (type-A0) makes it looks a lot like Sirius.
Vega is the brightest star of the constellation of LYRA. This is a pretty small constellation.
In this image
I've included the Big Dipper so you can get an idea of the scale
and the neighborhood.
Vega is in the spike on the top of the harp. The harp itself is made of four stars that form a nice diamond shape. | ![]() |
The two brightest stars in the diamond, Sulaphat and Sheliak, are on the side of the harp opposite Vega. They are easy to identify and they are important in our search because the Ring Nebula is positioned between them.
The Ring Nebula is a great "find" for amateur astronomers. All you need is a small telescope, a bit of practice and a lot of patience. Like I warned you before, it will not appear as beautiful as in the photo you saw earlier. Instead it will appear as a dim gray smoke ring.
What are those two bright stars in this evening's sky?
I think you are talking about Arcturus and Spica. You should learn both of these stars. They are very easy to identify.
First notice that the stars of the Big Dipper's handle form a
gentle arc. If you continue that arc, sweeping down and away from
the Dipper, you arrive at Arcturus. You can continue the
line directly to Spica.
I was taught to "Arc to Arcturus and speed on to Spica". That's a great way to remember it. Arcturus is the brightest star in the northern celestial hemisphere (magnitude -0.04). Hey I thought Sirius was the brightest! Sirius (magnitude -1.46) is at a declination of -17o but Arcturus is further north with a declination of +19o. Arcturus is light orange (type-K2) and 36 light-years away with a luminosity 115 times greater than the Sun's. It is a giant star although not a particularly big one. Indeed it is very similar to Aldebaran. (I plotted Aldebaran on the H-R Diagrams you have been studying, but not Arcturus.) Even though Arcturus and Aldebaran are orange, most astronomers would call them "red giants". | ![]() |
Spica is 257 light-years from us but has a luminosity over 2000 times that of the Sun. It is a bluish star (type B1) and would be plotted near Bellatrix on the H-R Diagram. Spica has a declination of -11o so you know that somewhere between it and Arcturus lies the Celestial Equator.
These two stars, Arcturus and Spica, are the brightest stars in two different constellations.
Arcturus is in the belly (or slightly lower) of BÖÖTES, the Herdsman. According to legend, Böötes invented the ox-drawn plough and was rewarded with his place in the stars. In Europe the Big Dipper is often called "the Plough". It looks as if Böötes is steering the Plough through the sky. Unfortunately, the official name for the constellation in front of him is URSA MAJOR, so we try to imagine Böötes is scaring a big bear. Of course the constellations are just the "official" way of conveniently imagining the sky but you can imagine what you like. Every culture has its own constellations. Indeed, in 1775 a famous astronomer (Bode) created a constellation called "QUADRANS" from the top of what we now call BÖÖTES. That name did not stick but the January meteor showers that radiate from this part of BÖÖTES are still called "the Quadrantids". | ![]() |
Spica is the brightest star in VIRGO, the Virgin. According
to Greek mythology, she was the daughter of Jupiter and Themis.
(With a father as powerful and frightening as Jupiter it's no
wonder she's still a virgin after thousands of years! )
Her real name was "Astraea" and she was the Greek goddess
of justice.
VIRGO is a huge constellation and doesn't look at all like a girl.
Instead, it is better to remember that Spica is at the
base of a Y-shaped part of the constellation and a series of very
dim stars make up the rest of VIRGO.
Do you recognize the constellation to the right of VIRGO, under the Big Dipper's bowl?
Yeah, it's Leo.
What about that group of stars on BÖÖTES right (my left)?
That's CORONA BOREALIS, the Northern Crown. It's a favorite among amateur astronomers because it contains quite a few binary stars and a recurrent nova!
We'll come back to these topics and this constellation in subsequent lessons. Until then, try to become familiar with this small but interesting constellation. In this image I've cranked up the magnification and increased the brightness to show you stars as low as 7th magnitude.
Alphekka (also known as Gemma) is the brightest star in the crown with a magnitude of 2.2. It is of spectral class A0 - and it's a binary star. Inside the bowl of the crown are two dim stars - M Coronae and R Coronae. M Coronae is close to a magnitude of about 7 so binoculars will be useful here. This star has a constant (dim) magnitude. R Coronae is an F-type star and usually has a magnitude of around 6, so if you can see M Coronae you should be able to see R Coronae too. But every now and then there is a BIG dip in the brightness of R Coronae caused by the sudden accumulation of clouds of soot in its atmosphere! | ![]() |
This unpredictable event can cause R Coronae to dim to a magnitude of 15 and it will remain dim (invisible to all but the best telescopes) until the soot is blown away by a gust of R Coronae's stellar winds. These kinds of stars, now called R Coronae-type, are very rare and R Coronae is one of the few that can be followed without optical aid (but a telescope makes it more fun).
By the way, there is probably not a protoplanetary nebula doing this. You would only get them around red giants (M-type).
Between CORONA BOREALIS and LYRA you will find HERCULES, a dim but very large constellation.
Four stars outline his keystone-shaped chest and two more stars give the outline to the bottom of his toga.
According to legend, Hercules was the
strongest man in the world. From the looks of his constellations,
he must have been quite a good dancer too! There are lots of interesting sights in HERCULES but first I want to point out something not in this constellation. | ![]() |
Rasalhague (magnitude 2.1) is also called alpha-OPHIUCUS. It's very close to Rasalgethi which is also called alpha-HERCULES. Just because they are close to each other in name and position doesn't mean they are in the same constellation. Indeed, Rasalhague's Bayer name (Alpha-OPHIUCUS) is your clue that it belongs to another constellation. OPHIUCUS is a very large, dim constellation below HERCULES and we will come back to it in a subsequent lesson.
Rasalgethi is a notable star because it is a red supergiant - probably bigger than Betelgeuse. Rasalgethi is 218 light-years away and has a surface temperature below 3000oC, which would place it on the far right of the H-R Diagram we have been using. The outer layers of this star are very "rarefied", meaning it has a very thin outer envelope. (This star has a "fluffy surface".) We see this star as red but in fact it releases most of its energy as infrared. We can't see infrared so its 3.0 magnitude is only part of this star's energy output. Note that this is the opposite end of the energy spectrum of the white dwarfs. White dwarfs emit most of their energy in the ultraviolet, so most of the white dwarf's energy is also invisible to our eyes. When Rasalgethi matures into a white dwarf it will shift its energy output to the other end of the invisible spectrum - from infrared to ultraviolet.
By the way, about every 3 months or so Rasalgethi's magnitude drops to as low as 4.0. It's classified as a "semi-regular variable star". Hmmm, a red supergiant, emitting mostly in the infrared, whose luminosity drops every now and then. Maybe this drop is due to the occasional puffing away of its outermost layers, producing protoplanetary nebula that temporarily block its light. Perhaps, but these variations are slow and we might expect these kinds of nebula outburst to cause sudden decreases in brightness. An alternative idea is that the star is just "pulsing" away like many old red giants do. Besides, the story of Rasalgethi is complicated by the fact that it has a small companion with a magnitude of 5.3. (We will return to the topic of binary stars in a few months.) This companion is very close to Rasalgethi - completing its orbit in 51.6 days. It is very likely that both stars are surrounded by a cloud of gas and tugging each other about.
More data will tell. Watch this space. Literally! This is an easy variable star to follow with the naked eye as it changes its magnitude over the course of weeks and its companion can be resolved from Rasalgethi with a small telescope.
HERCULES is the home of a globular star cluster called M13.
We discussed star clusters last month and I told you that to see
the best one would require a trip far south. M13 isn't the best
example of a globular cluster but it's just barely visible to
the naked eye - it has a magnitude of 5.9.
On a very clear, moonless night you should try to find it. If you have binoculars, or better - a telescope, you can find M13 by scanning along the line on HERCULES' left side. As you scan towards his armpit you'll find a very dim, diffuse glow. A small telescope will show you some of the individual stars in the outer zone of the cluster. | ![]() |
M13 is over 20,000 light-years away (!) and observations with very powerful telescopes show that it is full of stars of all ages. Remember, an open cluster will have drifted apart early in its "life" but a globular cluster is so big that it forms a strong gravitational attraction of its own, so it retains most of its stars "forever". M13 is between 100 and 200 light-years in diameter (depending on how you determine where its edge is).
I've introduced you to a few more constellations and it is important that you get outside and find them. Remember, the sky is always rotating around Polaris so some of my images show the constellations at different angles from the way you are likely to find them.
Use LEO and Capella to find Vega and LYRA. Even
if you don't have a telescope, imagine where the Ring Nebula is
located (between the two bright stars in the harp opposite Vega).
Use the Big Dippers handle to arc to Arcturus and speed
onto Spica. Then try to connect up the correct stars to
define BÖÖTES and VIRGO. Identify CORONA BOREALIS and with the help of binoculars and a good dark night, try to find both M Coronae and R Coronae. Indeed, every chance you get, try to find R Coronae. It might take a dive at any time.
If the sky is good (no clouds or moon) find HERCULES
dancing next to BÖÖTES. He is a dim constellation but his chest
is a good place to start. Identify Rasalgethi (alpha-HERCULES)
and Rasalhague (alpha-OPHIUCUS). You might want to take notes of how bright Rasalgethi is from night to night and maybe (just maybe) one day we will see it really blow its top!
Try to find M13 and, if you have binoculars, have a good look at
it. This globular star cluster is over 20,000 light-years away, the most distant
object we have talked about in these lessons (so far).
I hope this lesson has taught you some more stars and constellations. It's easy to get lost in the details of astronomy and forget that astronomy is fundamentally an observational science. We finish this month's lessons with the auroras. Unlike most astronomy, the auroras are hard to predict and complicated to understand but, after this next lesson, you'll know what causes auroras and how to make sense of them.