Size perspective on stars, planets and moons

Here's a rare opportunity to gain some perspective on celestial bodies, courtesy of Wikipedia.

In the first set of pictures, the order of magnitude successively builds up through our planets and different types of stars.

perspectives: Earth, planets, Sun, and other stars

NOTE: to view better, click on the image to expand, or see the original

(1) = Mercury, Mars, Venus, Earth;
(2) = tiny Earth against far larger twins Neptune and Uranus, then gas giants Saturn and Jupiter;
(3) = tiny Jupiter and red dwarf star Wolf 359, next to the Sun (a white main sequence G star), then, white main sequence A star Sirius A, near neighbour (8.6 light years) and our brightest actual star;
(4) = tiny Sirius next to orange giant star Pollux, then red giant Arcturus, then Aldebaran, another orange giant;
(5) = tiny Aldebaran and blue supergiant Rigel, next to red supergiants Betelgeuse and Antares
and finally
(6) = Antares, hypergiant S Doradus, red supergiant KY Cygni, and red hypergiant VV Cephei A*.


It's a very useful way to represent varying orders of magnitude. Resolution is better in the original, where the names are clearer*.

By contrast, another way of comparing them is by mass - this gives an idea of density rather than size (volume) above - this shows that some of the stars above are relative puffballs.  Solar mass is typically used: one solar mass (M_o is the mass of our sun, or about 2x10³⁰ kg.  Using that measure:
Wolf 359 (red dwarf) = 0.09 M_o
Sun (white G) = 1 M_o
Sirius A (white A) = 2 M_o
Pollux (orange giant) = 1.9 M_o
Arcturus (red giant) = 1.1 M_o
Aldebaran (orange giant) = 1.7 M_o
Rigel (blue supergiant) = 17 M_o
Betelgeuse (red supergiant)  = 18 M_o
Antares (red supergiant) = 15 M_o
S Doradus (hypergiant) = 45 M_o
KY Cygni (red supergiant) = 25 M_o
VV Cephei A (red hypergiant) = uncertain! - 25 - 100 M_o

Update 11-Aug-10:
A new star was announced in July, the most massive found yet.  Called R136a1, its mass is said to be 265 M_o - so that is now the current upper limit for mass.


Our nearest neighbours, Venus and Mars, are quite different from each other, in composition as well as size. Mars' radius is only about twice that of the moon. Venus is closer to the sun than Earth; its oceans were boiled away 4 billion years ago. Mars has traces of ice, but no liquid water, as its atmosphere is (now) too thin.

It's worth scanning through the full originating article on stars, for some points of interest. Our sun is quite small compared to other stars, but there are some that are smaller (and denser), having burnt out and collapsed. The article also lists the various types of star by name, but actual stellar classification is much more complicated.

The other picture shows various moons of the solar system in scale against Earth.

Moons of the solar system, compared to Earth
for better resolution, click on the image to expand, or see the original

This includes the two biggest moons, Jupiter's Ganymede (rock and ice, often featured in science fiction stories) and Saturn's Titan (dense atmosphere, surface liquid), which are both larger than the smallest planet, Mercury (which is slightly larger than the third biggest, Callisto at bottom). Our moon is the largest of any rocky planet in our system, having broken off from Earth early in planetary formation after Earth was hit by an object of about the size of Mars.


*You can also look at the current Wikipedia holding on relative sizes, which shows our system's full planet range.  You can also scan through the Wikipedia list of most massive stars.  Note the discussion there, indicating there remains some uncertainty at the big end (which is largely theoretically inferred), to the extent that the various Wikipedia articles and pictures have inconsistencies.

Practical astronomy lessons

I was never interested in astronomy: those patterns of stars never made any sense to me. But recently I spoke to an astronomer, and he helped put a lot of pieces together for me.

Stephen works for the Anglo Australian Telescope, a joint venture between those two governments (although it looks like the English are planning to pull out).

The telescope itself is situated near Coonabarabran, in the west of New South Wales; however Stephen is located in Sydney, sending requests out west, receiving data back, and sometimes travelling out to be there in person. In stark contrast to me, he had always loved astronomy, has a PhD, and seems to be working where he wants to be. he painted several dichotomies which helped me understand the discipline. He was really on top of his subject, so any error here is due to my effort in understanding.

One of those dichotomies is between the practical and the theoretical astronomer. The practical astronomer will do all the practical work in aligning the telescope, identifying the items of interest, and taking the images. The work is then handed over to a theoretical astronomer, who tries to work out what it all means. If anything, I would be that theoretician; the practical side would scare me, but the maths and physics would fascinate.

He also clarified for me the difference between optical and radio telescopes. Although I should have had an inkling, I never did join the dots: there is quite a distinction between the two. The first, of course, scans space for what can be seen in visible light, using what we traditionally understand a telescope to be, lenses and all. But a radio telescope picks up radiation in the spectrum beyond ultraviolet light, thus needing non-optical radio receivers to pick up data from space. They're both picking up information from the EM spectrum, but representing and analysing it differently. Radio telescopes can be as in the film The Dish, which has an enormous parabolic dish that concentrates signals to a point which records them.

This is the Parkes observatory (again in western New South Wales), which is part of the CSIRO (Australia's scientific research organisation) whereas the AAT is not part of the CSIRO at all.

Radio telescopes can also be set up in an array of smaller ones, as in the film Contact - in fact, there is a site in Western Australia that is in the running for global funding to develop an array of dishes that would be one of the largest in the world, running to an area of square kilometres.

Stephen said that, in a very broad sense, optical telescopes took images that represent a (relatively) static view of the universe - as it was at a single point in time (so to speak - the light from distant objects, of course, would date back further in time than that from closer objects). In contrast, radio telescopes recorded data from events - a more dynamic picture being built up of suns flaring, galaxies colliding, black holes, gas clouds, etc. My guess is that images we see that are built from radio telescopes are colour coded to depict the intended narrative that is not otherwise representable visually, whereas optical images may at times be enhanced to also provide clarity to the narrative. Unfortunately, this means some people would be misled into thinking that at a macro level, the universe is more colourful than it really is.

Stephen told me much more about the subject than I have reproduced here, and I am very grateful to him for helping me fill so many gaps in my understanding.

Coincidentally, my family (without me) passed by Parkes a couple of weeks after our conversation, which is why I have this magnificent picture. The dish is 64m in diameter, covers 3216 square metres, and weighs 300 tonnes. Operating frequencies range from 440 MHz to 23 GHz.

Tech: identify the stars

SkyScout is an amazing device: it identifies stars.

It looks rather like a small video camera. Target it at any given star, and it will identify what it is. You can also locate a star or planet by selecting the name, then moving the device in the direction of the guiding arrows.

Astronomy - for me - is simply a confusing mass of bright spots in the sky. This is the sort of device that can help the astronomically challenged. Perhaps, for some, it may propel more people towards a more fundamental and fascinating subject: cosmology.