Someone recently wrote to me about misstatements in the media. In particular, a TV reporter had referred to the sun as the "brightest star known." Of course this is absurd, but in fact the sun is the brightest star in our sky. This got me to thinking. How bright does the sun appear from the other planets? In particular, is it just a "bright star" as seen from (the now "dwarf planet") Pluto? What follows is a revised version of an article I wrote some years ago.
[Image at left of the New Horizons spacecraft passing Pluto and Charon, with the distant sun in the background, in 2015.]
As with any luminous body, the intensity of radiation of the sun diminishes in accordance with the "inverse square law." This means that as the distance between the observer and the object increases, the relative "brightness" of the object goes down very rapidly — "exponentially" as we say.
Another measure of "brightness" is called magnitude. The system arose in ancient times by which the brightest stars were first magnitude, the next brightest were second magnitude and so on. This was a fairly imprecise system that was later modified into a more mathematical format in which each lower number was 2.512 times brighter than the next higher number. Thus, a first magnitude star is 2.512 times brighter than a second magnitude star; a second magnitude star is 2.512 times brighter than a third magnitude star and so on. This works out such that a difference in five magnitudes is the equivalent of 100 times in intensity.
Originally the brightest stars were considered first magnitude (+1), but more modern astronomers realized that there were stars brighter than first magnitude. Thus we have 0 magnitude and even negative magnitudes (Sirius is -1.4). Here we are considering visual light only, so these are called visual magnitudes.
Taking this into consideration, I constructed a small table:
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | |
| Distance in AU | Planet | Brightness ratio | Magnitude | Arc seconds | Ratio to Moon | Ratio to Venus | Ratio to Sirius | |
| 1 | 0.39 | Mercury | 6.68E+00 | -28.81 | 4651 | 2.78E+06 | 4.41E+09 | 8.88E+10 |
| 2 | 0.72 | Venus | 1.91E+00 | -27.45 | 2490 | 7.97E+05 | 1.26E+09 | 2.55E+10 |
| 3 | 1.00 | Earth | 1.00E+00 | -26.75 | 1800 | 4.17E+05 | 6.61E+08 | 1.33E+10 |
| 4 | 1.52 | Mars | 4.31E-01 | -25.84 | 1181 | 1.79E+05 | 2.84E+08 | 5.73E+09 |
| 5 | 5.20 | Jupiter | 3.70E-02 | -23.17 | 346 | 1.54E+04 | 2.44E+07 | 4.92E+08 |
| 6 | 9.55 | Saturn | 1.10E-02 | -21.85 | 188 | 4.57E+03 | 7.24E+06 | 1.46E+08 |
| 7 | 19.20 | Uranus | 2.71E-03 | -20.33 | 94 | 1.13E+03 | 1.79E+06 | 3.61E+07 |
| 8 | 30.16 | Neptune | 1.10E-03 | -19.35 | 60 | 4.58E+02 | 7.26E+05 | 1.46E+07 |
| 9 | 39.81 | Pluto | 6.31E-04 | -18.75 | 46 | 2.63E+02 | 4.17E+05 | 8.39E+06 |
| Col. 1 Col. 2 Col. 3 Col. 4 Col. 5 Col. 6 Col. 7 Col. 8 |
— Average distance of the planet from the sun, relative to Earth — Planet — Intensity of sunlight, compared to Earth — Apparent visual magnitude of sun at planet — Angular diameter in arcseconds — Brightness ratio compared to the Moon — Brightness ratio compared to Venus — Brightness ratio compared to Sirius |
What emerges from this are a couple of interesting facts. The first has to do with angular diameter. From the Earth, the sun subtends an angle of about 1800 seconds, or about a half degree. From the distance of Saturn. the sun appears just barely more than a tenth of a degree across. At the average distance of Pluto, the sun is about 46 seconds of arc, less than one minute and roughly the size of Jupiter as viewed from Earth at its best. Thus, from Pluto it is true that the sun would be essentially starlike in terms of size. No disk would be seen without a telescope. However, brightness is another matter altogether. Even from the distance of Pluto, the sun would shine at a magnitude brighter than negative 18, and some 263 times the brilliance of the Full Moon as seen from Earth (Column 6, Row 9)!
Further investigation shows that at the distance of Pluto the sun is still 8.4 million times brighter than Sirius, the brightest star in Earth’s skies! So, to say that the sun as seen from Pluto is little more than a bright star is very misleading!
Note: the figures in the table are estimations, but are essentially correct.
Image Credit: Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute (JHUAPL/SwRI)
Copyright 2007 Final Copy, inc.
Very interesting! I sometimes see artist’s depictions of the sun from the surface of Pluto, which don’t convey the amazing brightness of the sun - as seen in a starlike point - from that distant world.
As for the statement in the media that “the sun is the brightest star known,” of course that’s just a matter of the reporter not really knowing or thinking very hard about what he’s saying.
Teachers sometimes do this, too. I recall going into my child’s preschool class more than 20 years ago and hearing the teacher say to the kids, “The sun is a great big ball of gas, and the stars are little bitty balls of gas.” I know what she meant! But she didn’t quite say what she meant to say.
And in fact it’s pretty easy for those of us who work in science writing to say the wrong thing … even when we know better. In writing about astronomy, I sometimes say things like “Venus is now farthest from the sun,” forgetting to add “on the dome of Earth’s sky.”
In writing or speaking about science, we need to be very precise! Otherwise, it’s easy to convey a wrong impression.
“My opinion and welcome to it!”
I think it is very difficult if not impossible to portray the very small size and intense brilliance in a painting, or for that matter on a planetarium dome or computer screen. What often happens is that time image is made physically bigger since it is impractical or impossible to make it that much brighter. Sometimes the power of numbers is the best solution to explain something we can’t experience directly or even in some cases fully imagine.
As for making mistakes, that’s about as easy as breathing. I make more than my fair share. But reporters — and I am referring here mostly to TV — are often little more than talking heads. Truth is, they can’t be experts in everything and because of that, they often make mistakes. But the biggest mistake is in not checking their facts. Honest mistakes or slips of the tongue are one thing, but I think the biggest and worst mistake a reporter can make (other than blatantly fabricating “facts”) is to fail to check the sources and confirm their information. Too many just read what was given them or parrot something some equally uninformed person told them.
Of course there are many exceptions, and much of it can be attributed to the excessive pressures and time constraints they work under, but I think the phrase “journalistic integrity” has lost much of its meaning these days.
And as for teachers, certainly they can and often do say wrong things. In your case with the preschool teacher, maybe you DID know what the teacher meant. But in my experience, I would say that many, MANY times teachers make such mistakes not just because they misspoke, but because they are misinformed or suffering from misconceptions. I don’t mean to sound negative, nor to ignore the many mistakes I make on a regular basis, but too many people today simply do not think clearly or adequately.
LS
Please read my last post titled “Globalwarming Awareness2007 in the Universe” at my
Globalwarming Awareness2007 blog. You will see that ANTARES is the brightest star in the Universe.
MES
MES,
Yes, I’ve seen those photos before and they seem to be reasonably accurate. However, based on absolute magnitudes, Antares is one of the brightest stars, but certainly not *the* brightest star in the Universe. In any event we can only speak of stars we know of, and there are many, many stars in other galaxies that even our best telescopes cannot image.
One of the most massive stars known in the galaxy is Eta Carinae. Not sure how that massiveness translates into brightness, exactly, but Eta Carinae must be among the galaxy’s brightest stars …
Another candidate for “most massive star” has been Pismis 24. I just edited an Earth & Sky radio show about it, which we’ll be recording next week. It’s recently been found to be not one but several stars, each with about 70 times the mass of our sun. That’s been suspected of Eta Carinae, too … that it’s not one star, but several stars.
The upper mass limit for stars, theoretically, is still somewhere around 150 solar masses, according to research done in the past few years.
If you consider absolute visual magnitude (that is, visual brightness), then there are several that outshine Antares — Rigel, Deneb, Almaaz. By my sources, Eta Carina has an absolute visual magnitude of about -12.1. Another, Cyg OB2-12, is about about -12.2.
Currently the star some consider brightest is the “Pistol” star in the “Pistol Nebula,” which is thought to outshine our sun by 10 million times. Its absolute visual magnitude is about -12.3.
At -5.8, Antares is nowhere near this bright.
However, the most luminous star known appears to be LBV 1806-20, which is more luminous than the sun by up to 40 million times, and may have an absolute visual magnitude of -14.2! There is a table on Wikipedia:
List of most luminous stars
L
Of course, if the sun were the brightest star known (the error you mentioned, Larry, in the first paragraph of this post), it would violate the “Cosmological Principle”:http://en.wikipedia.org/wiki/Cosmological_principle: the idea that we are not in a preferred place in the universe.
Unless, of course, a precursor for human life were that it needed the brightness of the galaxy’s brightest star in order to evolve!
I sense a science fiction story idea in the making …
Deborah
Well, I don’t think being near the brightest star in the Universe would in any way imply a violation of the cosmological principle. The basic or original cosmological principle just specifies that the universe have the same bulk properties no matter where you look. That cannot be absolutely true or there would be no concentrations of matter — no stars, no planets, no us. If we required that it be perfectly isotropic and homogeneous, how could there be objects or the large scale structure of the Universe?
So we can be near the Sun or any other star and it wouldn’t necessarily be a violation as long as the bulk properties of space even out over large distances in all directions. Even if we were orbiting the brightest star in the Universe, that could be pure coincidence. Some star has to be the brightest, right? Surely that does not imply a violation in itself. I am assuming that there is a brightest star in the Universe, but not that it occupies any preferential location.
Now, if we were the only life in the Universe, and we orbited the truly brightest star, that might say something. But we could never be sure. We could no more prove that we were the only life in the Universe than we can prove that God doesn’t exist. Just can’t be done.
L.
i very much enjoy this blog . mind boggling when you think of the distances sizes and brightness of stars. i wish (if my eyes were able to stand it and i didnt die of the bombardment by particles and radiation)…anyway i wonder how reathtaking it would be to see the sun from the surface of mercury? just the size alone filling the horizon would probably be hypnotic. thanks all for providing all of the thoughts and facts.
Hi Larry,
Today I get new information that VY Canis Majoris (VY CMa) is a red hypergiant star.
Please check these links
the largest known stars and one of the most luminous stars known.
MES
Hi, Sam. Glad you enjoyed it. Actually, the sun is not that big as seen from Mercury. The sun is about a half-degree across as seen from earth, and Mercury is roughly one-third the earth’s distance from the sun. So from Mercury, the sun would be roughly a degree and a half across. On the average it should be, oh, roughly the size of an orange held at arm’s length.
Thanks for the links. I’ll check them out!
Larry S.
oh. well thanks, i thought it was closer. distance and size on a cosmological scale are simply stunning to even imagine.
Larry,
Your article was fascinating. However, since part of the article focused on accuracy in the media, I do have an accuracy quibble. I’ve been taught differently about the meaning of “exponential” variation.
The article states that the light intensity diminishes as the square of the distance, and this is called “exponentially.” I don’t believe that that is accurate. Varying as the square of distance or the hundredth power of distance is not considered exponential. To be exponential, the variable has to itself be part of the exponent. With light intensity versus distance, the distance is at the base, and a constant, 2, is in the exponent. In other words, light intensity varies as d to the power of two, or I = d^2.
If it were the other way around, if the light intensity from a point source varied as two to the power of the distance, I = 2^d, then the variation would be exponential. (There is nothing magic about the 2, it could be 3^d, e^d, or any other number of course.)
For example, on Saturn, which is ten times the distance from the sun that the earth is, the sun’s apparent intensity would be 10^2, or 100 times less than on earth. If the variation were exponential, the ratio of intensities would be 2^10, or 1024.
The classic story of the inventor of chess, who asked that a single grain of wheat be placed on the first square of the checkerboard, and then doubled on each of the remaining squares, was trying to take advantage of exponential growth. The final square would have had 2^63 grains, or roughly 9 with 18 zeros after it. As the story goes, the king realized he’d been conned long before they got to square 64, and had the guy beheaded.
That minor misstatement didn’t affect any of the numbers in the table or the conclusions of the article, of course. I just couldn’t resist.
Ray
i want to start by saying this may sound stupid and im not a cosmologist, astronomer, etc. i just wake up each morning wanting to know more than i did yesterday. anyway..the dictionary defines a star as any light emitting body. a blackhole has an accretion disk wich puts out light thru…you know..amyway to keep this short.. does a black hole fall at the top of the h.r.diagram or the bottom?..if it in fact is even considered to be a star …it may not i dont know and i wont be offended if you just ignore this question..thank you
Larry,
I’m finally getting around to checking out the new blogs. Very much enjoyed your “How bright is the sun?” Just want to make a quick comment on the sun’s size as seen from Mercury. Even though the sun’s angular diameter would be some 3 times larger than here on Earth, it’s disk size would be 9 times larger (diameter squared).
That would probably be pretty impressive!
Bruce
RE: Ray Cobb,
Thanks, Ray. You are correct, but my sloppy use of the term was to imply a very rapid diminishing of brightness.
Larry S.