Thread: Star Trivia....
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Old 29-02-2008, 12:35 PM
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ngcles
The Observologist

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White/Orange dwarfs -- further info & Trivia

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

Last edited by ngcles; 29-02-2008 at 12:47 PM. Reason: Correction
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