Star Trivia....

[Solanum]

....to which I don't have the answers!

I was wondering today, which is the brightest known white dwarf, intrinsically and/or visibly I guess.

I imagine that Alpha Centuri B (if it is still designated B?) is the brightest white dwarf visually, but it's pretty hard to see with it's rather bright companion and I have no idea whether it is bright for a white dwarf intrinsically.

Any answers?

[Blue Skies (Jacquie)]

Alpha Centauri B is a orange K class star. I think you're thinking of Sirius B? But I don't have an answer for you on intrinsic brightness sorry.

[Solanum]

Oops! I certainly was thinking of Sirius B, you're totally right!

[ngcles]

Hi Solanum & All,

I'm pretty sure that the brightest white dwarf is Sirius B with an apparent visual magnitude of 8.44 --it is also the closest to the Sun at only 8.4ly.

The third brightest and 2nd closest is Procyon B at 11.4 ly and with an apparent visual magnitude of 10.82.

Both of these white dwarfs are quite/very hard to observe visually because of their close proximity to their brilliant companion stars. Procyon B in particular is very, very nasty (I believe, I've never seen it or even suspected seeing it). The seperation _at very best_ is 5 arc-seconds, and the magnitude difference between it and Procyon A is 10.5 magnitudes. It is near imposible to observe unless it is at periastron and the telescope is both very large and high quality and with superb seeing and an experienced observer at the business end of the 'scope. There are very, very few reliable amateur observations of Procyon B.

From memory, probably the "easiest to see" white dwarf is in the Omicron2 Eridani (40 Eridani) system, which is otherwise known as "Keid".

It is a local triple-star only 16.4ly distant consisting of a magnitude 4.4 K-type main-sequence dwarf that is one of the few stars of its class that is visible to the naked eye.

I can presently think of only 5 (well, 6 technically) other K-type main sequence dwarfs that are visible to the unaided eye. Only two of them are in a single star system -- and one of those (I believe) is intrinsically the dimmest single star that is visible to the naked eye. Hmmm ...

The other two members of the Omicron2 Eridani system are a distant 83 arc-seconds away. They consist of a magnitude 9.5 white dwarf (the 3rd closest white dwarf to the Sun -- and most easily seen) which is in turn circled by an easily resolved magnitude 11.5 M-class red dwarf, as it is a wide 8 arc-seconds distant. The three are plain as plain in an 8" newtonian at low power.

You can read all about Omicron2 Eridani here:

http://www.astro.uiuc.edu/~kaler/sow/keid.html

So, in one naked eye system, the smallest 'scope can show three very interesting (and visually infrequently seen/noted) stars!

The intrinsically brightest white dwarf? Hmmm ... that is a real toughie. I really haven't got the foggiest (Yet)!

Best,

Les D
Contributing Editor
AS&T

[Solanum]

Wow, thanks Les! A font of knowledge. I shall certainly look for Omicron2 Eridani when I get the chance, it sounds like a rather interesting system. I have looked for Sirius B a couple of times, but have yet to get more than a very slight (and possibly imagined) hint.

A couple of questions to follow up:

1) Is there a good source for answering this type of question, or is it a matter of experience?

2) I guess it must be close to 20 years since I read up on stellar evolution (H-R diagrams and the like), and knowledge must have changed a fair bit since then. Does anyone have a good book they can recommend?

[ngcles]

Hi Solanum & All,

After a bit of a further poke around this morning I can add a little to the above.

Omicron2 Eridani is in fact the 4th closest white dwarf to the Sun (not the 3rd as I said above). 3rd is Van Maanen's star in Pisces -- 14.4 ly distant. It is the closest single white dwarf to the Sun (the others are in binary or triple star systems.

Omicron Eridani was the first proven white dwarf discovered -- in 1910. Sirius A was discovered earlier by Clark in 1862, but not proved to be a white dwarf until 1915.

Of stars in the Solar neighbourhood, about 5% are white dwarfs.

The intrinsically brightest white dwarf? I have an answer but it is not an entirely satisfactory one (it is a "trick answer" actually).

The intrinsically brightest white dwarf (albeit very briefly) are the Type Ia supernovae which are caused by the catastrophic detonation of a Carbon-Oxygen white dwarf when it exceeds the Chandrasekhar limit of 1.38 solar masses. The usual mechanism to achieve this mass-gain is via accretion from a companion star. The Type Ia events have an essentially uniform absolute magnitude of -19.3 -- somewhat brighter than the Type II events caused by core-collapse of a massive star.

Further to the above, after a bit of further digging I can come up with a list of 14 K-type main sequence dwarf stars that are visible to the unaided eye (some are only barely visible). There might be one or two more I haven't dredged up, so the list is not exhaustive. K-dwarfs are probably the rarest naked-eye stars that comprise about 0.2% of the 6,000-odd stars that can be seen naked-eye. Some are in a system where it is paired with a brighter but dissimilar star -- like Alpha Centauri A & B. In some cases they are paired with smaller but dissimilar stars -- like the Omicron2 Eridani System, in a couple of cases we have K-type dwarfs paired together and many singles. The brightest is of course Alpha Centauri B -- a K0 V dwarf that would, in the absence of Alpha Centauri A, just scrape into the 1st magnitude category all by itself at mag 1.4. To make my (so-far) list of 14, the K-type star must be sufficiently bright to be visible to the naked-eye in the absence of any brighter companion.

But, what is the intrinsically dimmest single star that can be seen with the unaided eye?

Some may argue that it is 61 Cygni A+B which is a pair of K-type dwarfs with 15% and 9% of the Sun's luminosity respectively. Combined, they have about 24% of the Sun's output. But, among that list of 10, there is a single star which is probably marginally less luminous that the combined effort of 61 Cygni. It is Epsilon Indi -- only 11.8ly away it is a K4-5 Ve star with 77% of the Sun's mass but only 22% of its luminosity. TW Piscis Austrinus is also a candidate of least luminous single star.

That list of 14 K-dwarfs visible (or would be if alone) to the naked eye, not in any particular order is:

Alpha Centauri B
61 Cygni A+B
70 Ophiuchi A+B
Sigma Draconis
Gamma Leporis B
Epsilon Eridani
Omicron2 Eridani B
Epsilon Indi
12 Ophiuchi
107 Piscium
p Eridani A+B
TW Piscis Austrinus

If you can think of any to add to this list, add them!


Best,

Les D

[ngcles]

Hi All,

Adding a few more to the list of 14 K-type naked eye dwarf stars that must be sufficiently bright to be visible to the naked-eye in the absence of any brighter companion we have:

Gliese 570 A (Libra)
36 Ophiuchi A+B+C
Gliese 783A (Libra)
Gliese 105A (Cetus)
107 Piscium
Chi Draconis B
12 Ophiuchi

So the list is now 23 naked-eye K-dwarfs, but nearly all of these are very faint stars and most are only barely visible naked eye, comprising, at best, 0.3% of the naked-eye stars. They are rare to see!

As a matter of interest, within 10pc of the Sun (32.6ly) there are in total:

4 A-type stars (Sirius, Vega, Altair, Formalhaut)
8 F-type stars
19 G-type stars of which the Sun (I think) is intrinsically 3rd brightest
43 K-type stars, and
>263 M-type stars
> 22 white dwarfs

There are no stars of true giant (III) or supergiant (I) luminosity class. Excepting the white dwarfs, they are all main-sequence stars (V) with only a couple displaying in their spectra the possibility of being evolving sub-giants (IV) (Procyon is an example).

So, of the total of at least 359 stars (probably more like 380-400 counting the dimmest undiscovered red and white dwarfs), only 14 are brighter than the Sun and 95.8% are dimmer.

That makes the Sun look pretty impressive don't it!

Best,

Les D
Contributing Editor
AS&T

[Solanum]

That's a pretty interesting list, though it doesn't give me much hope of seeing many white dwarfs! I shall be looking out for Eridani B though. On the subject I came across this paper:

http://arxiv.org/abs/0707.1343v1

Which includes a table of the brightest white dwarfs in the Tycho-2 catalogue. (table 2). Now if only I knew how to translate the WD number in to something I have any idea of.....

[ngcles]

Hi Solanum,

I assume you are referring in the paper to the short table of bright white dwarfs.

The first column is the WD (or white dwarf) catalogue No for selected stars.

Use that number in the Arizona database here:

http://procyon.lpl.arizona.edu/WD/

And it looks like you will get as much info as you need/want. The pages there are still under construction though! Some catalogue entries seem to load quick and complete others not so.

You could also do a Google search with WD catalog white dwarf star (make sure you use the US spelling of catalog) and it will bring up some other sources that include the WD catalogue.

Regarding some of the positions given,, beware that some are in the J1950.0 epoch, look for and alternate J2000.0 position or be prepared to do some precession calculation!

Hope this is of some help.

Best,

Les D