Starship Asterisk*

So which of the stars on the list is intrinsically the brightest?

The prize for the brightest star in optical light of the 25 on the list should probably be shared between Rigel and Deneb. Rigel, spectral class B8 Ia, has a visual luminosity some 49,300 L_☉. But because Rigel is a B-type star, albeit a cool B-type star, it produces quite a bit of ultraviolet light, pushing its bolometric (total) luminosity to 85,000 L_☉.

Deneb, spectral class A2 Ia, is probably as bright as Deneb in visual light. Its visual luminosity is 49,000 L_☉, but at the distances we are talking about (some 1,400 light-years for Deneb as measured by Hipparcos), 49,300 L_☉ is the same thing as 49,000 L_☉, in view of the margins of error here. In optical light, Rigel and Deneb are equals. But because Deneb is considerably cooler than Rigel it doesn't produce as much ultraviolet light as the Orion luminary, and Deneb's bolometric luminosity is probably "no more" than some 54,000 L_☉.

The brightest star on the list, when you take its bolometric (total) luminosity into account, may be Betelgeuse. Its optical light, according to Hipparcos, is some 13,000 L_☉. But Betelgeuse is a huge and very cool star, and it produces enormous amounts of infrared light. Jim Kaler estimated that the bolometric luminosity of Betelgeuse might be 85,000 L_☉, just like Rigel, but it might be more, up to perhaps 100,000 L_☉ or possibly even more.

I should add that Antares, while visually no contender of Deneb (some 10,000 L_☉ in optical light versus some 49,000 L_☉ for Deneb), is probably outshining Deneb in bolometric light, some 60,000 L_☉ versus perhaps 54,000 L_☉ for Deneb.

Ann

Wow Ann, composing that (2nd previous) post took a massive amount of work. A question though, what is your ranking basis?

BDanielMayfield wrote: ↑Wed Jun 26, 2019 2:28 pm Wow Ann, composing that (2nd previous) post took a massive amount of work. A question though, what is your ranking basis?

Well, apart from my software Guide, I rely almost entirely on Jim Kaler's Stars.

Ann

Ann wrote: ↑Wed Jun 26, 2019 7:55 pm

BDanielMayfield wrote: ↑Wed Jun 26, 2019 2:28 pm Wow Ann, composing that (2nd previous) post took a massive amount of work. A question though, what is your ranking basis?

Well, apart from my software Guide, I rely almost entirely on Jim Kaler's Stars.

Ann

Ok, that's your source. (I just use Wikipedia, expecting it to be fairly up to date.) But what I meant was, on what basis were you ranking them. Upon rereading your comment I see it was from least intrinsically bright upwards. I was being intrinsically dull in not realizing that at first.

BDanielMayfield wrote: ↑Wed Jun 26, 2019 8:58 pm

Ann wrote: ↑Wed Jun 26, 2019 7:55 pm

BDanielMayfield wrote: ↑Wed Jun 26, 2019 2:28 pm Wow Ann, composing that (2nd previous) post took a massive amount of work. A question though, what is your ranking basis?

Well, apart from my software Guide, I rely almost entirely on Jim Kaler's Stars.

Ann

Ok, that's your source. (I just use Wikipedia, expecting it to be fairly up to date.) But what I meant was, on what basis were you ranking them. Upon rereading your comment I see it was from least intrinsically bright upwards. I was being intrinsically dull in not realizing that at first.

Well, I was trying to answer a question from Evenstar. It was my impression that Evenstar asked how the Sun compares with 25 stars on the list. Then I got interested and decided to make a list. I wanted to show Evenstar that all the stars on the list are intrinsically brighter than the Sun. And when I ranked the 25 stars I decided to start at the bottom, because it is easiest by far to start with the least luminous ones. They are very nearby usually main sequence stars whose brightness and distance we can be sure of, and whose mass can reasonably be estimated.

Ann

Well, okay. I ranked the 25 stars on the list according to their intrinsic luminosity, starting with the least luminous ones. This is my list so far:

1) Rigil Kentaurus

2) Procyon

3) Altair

4) Fomalhaut

5) Sirius

6) Vega

7) Castor

8) Pollux

10) Capella or Arcturus

11) Arcturus or Capella

12) Regulus

13) Aldebaran

14) Gacrux

15)

16)

17)

18)

19)

20)

21)

22) Deneb or Antares

23) Antares or Deneb

24) Rigel/Betelgeuse

25) Betelgeuse(?)


Okay. Betelgeuse, Rigel, Antares and Deneb are true supergiant stars of luminosity class Ia. None of the other stars on the list are supergiant stars. But why not try to make the list complete?

First I will have to check out the remaining stars, Canopus, Achernar, Hadar, Acrux, Spica, Mimosa, Adhara and Shaula, with the help of my software Guide. I note that with the exception of Canopus, all the remaining stars belong to spectral class B. That makes it a bit easier to compare them. It is at least moderately okay to start by just comparing their intrinsic optical luminosities (as calculated by their apparent luminosities and their Hipparcos distances) and then assume that their "bolometric correction", when you take their total energy production into account, will be at least reasonable similar.

So. According to my software Guide, the optical luminosities of the seven remaining B-typ stars are as follows: Achernar, 1004 L_☉. Hadar (which is a triple star), 6850 L_☉. Acrux (also a triple star), 3990 L_☉. Spica (a double star), 1980 L_☉. Mimosa (a mostly "single" star with a few puny companions), 1920 L_☉. Adhara (mostly single, with a puny companion), 3220 L_☉. Shaula (a double star), 5700 L_☉.

So how do we make sense of these B-type stars as to how intrinsically bright they are? Bear in mind that their calculated luminosities are critically dependent on the Hipparcos measurements of their distances, and the Hipparcos margin of error is large at the distances of these faraway stars.

Anyway. According to Wikipedia, Achernar is the faintest of these seven B-type stars. All the rest are spectral classes B0, B1 or B2, but Achernar is spectral class B3 (B6 according to Wikipedia). According to Wikipedia, the luminosity of Achernar is "only" 3,150 L_☉, and that includes ultraviolet light. All right, Achernar is number 14 on my list.

I have to confess that the other six B-type stars drove me crazy. They are all very far away, the distances to them are uncertain, and new calculations by other sources than Hipparcos seem to have assigned new distances to some of them, some of them are multiple stars with very bright companions, and the confusion of M_v (total visual light) versus M_bol (total energy output) is more than enough to make me confused as to how to list these stars.

But anyway, here goes. Spica follows after Achernar on my list. After Spica comes, maybe, Shaula. After Shaula comes, if you consider the brightness of the individual stars only, Acrux, but if you consider the total output of the multiple star system, Mimosa. The next one is the one I didn't pick for number 17, Acrux or Mimosa. I'm going to pick Hadar as number 19 on my list, and number 20 vill be Adhara.

So where do I put Canopus? The thing about Canopus is that it is very visually bright, some 13,000 L_☉ according to my software. But according to Wikipedai it's only 10,700 L_☉. Also, because Canopus is an F-type star, it emits most of its light in the visual range of the spectrum, and its "bolometric correction" is very small. What you see is what you get from Canopus. So, if we consider bolometric luminosity, Canopus should probably be number 15 on my list, just after Achernar, but if you consider visual luminosity, Achernar is among the top 5, just after Deneb.

My goodness! Okay, here is my list, and now I'm starting from the top.

1) Betelgeuse (bolometric energy output) or Rigel (bolometric energy output).

Or: 1) Rigel and Deneb (tie): Visual light.

3) Antares (bolometric light).

5) Canopus (visual light).

Or: 5) Adhara (bolometric light).

6) Adhara (visual light).

7) Hadar.

8) Mimosa or Acrux.

9) Acrux or Mimosa.

10) Shaula.

11) Spica or Canopus (bolometric light).

12) Achernar.

13) Gacrux.

14) Aldebaran.

15) Regulus.

16) Arcturus.

17) Capella.

18) Pollux.

19) Castor.

20) Vega.

21) Sirius.

22) Fomalhaut.

23) Altair.

24) Procyon.

25) Rigil Kentaurus.


Okay, that's it! That's my ranking of the 25 stars in the APOD according to their sometimes visual and sometimes bolometric magnitude. Sue me!

Ann

Think of all the exoplanets missed all over our galaxy because their planetary plane is perpendicular (or nearly perpendicular) to Earth.

Evenstar wrote: ↑Sun Jul 07, 2019 6:38 am Think of all the exoplanets missed all over our galaxy because their planetary plane is perpendicular (or nearly perpendicular) to Earth.

Of course, systems that depend upon transits for detection are sensitive to geometry (although we have a large enough sample to know how many other stars also have planets, based on statistics alone). But we are starting to put into place systems that can image exoplanets directly. Those will allow us to identify planets in systems that aren't nearly edge-on to us.

I love this very creative and didactic photography of Tragoolchitr Jittasaiyapan, I congratulate you for choosing it as an apod;) I belong to an astronomy group where we make public observations, people often think that a bright star like Sirius must be the largest and as you know they are very wrong, so I want to draw a picture of these stars with their estimated size to scale and I need the data, from a serious and rigorous source, of the equivalence with the solar radius of these 25 stars (for example Sirius 1,711 R☉) . I have tried to find the information in SIMBAD but I have not known how to do it. Do you know any online page with this type of star information? Thanks a lot!