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Monday, August 08 2022 @ 12:35 pm EDT

Scale of Our Galactic Neighborhood

General NewsTo an ant, we're giants. To an amoeba, the ant is a giant. If we go the other way we feel like the world is huge until we consider what Mother Earth looks like beside Jupiter, and Jupiter seems pretty small next to our Sun, which we call Sol. Let's take a fun little joy ride around our local neighborhood in this arm of the Milky Way Galaxy. Be prepared to feel, well, insignificant by the time we're only a thousand light-years from home.

The rocky planets of our solar system, from Earth to lowly Pluto. This is but the starting point in a little tour of our local star out to around 1,000 light-years away to the red giant star Arcturus.
To our fellow creatures on Earth, our home seems vast and large. And yet I'm constantly imagining how small we are compared to other objects in our solar system, like Jupiter and Saturn. Its one of the more fun things we can use to help the uninitiated in astronomy to get a sense of scale from the very, very large to the very, very small. Its easiest for our visitors to start with how big we are, as humans, and how big other things are compared to that.

Consider the relative sizes of the 5 so-called "rocky" planets within our solar system, these being Earth, Venus, Mars, and Mercury. I include Pluto even though its technically an outer planet, due to it being a non-gaseous, non-giant planet. Even our own moon is larger than Pluto, but we're only comparing sizes of the nine planets for this article.
Comparing relative sizes of the inner planets to the outer planets. Are we feeling small yet? Not quite.....
This is the only time we'll feel like our own Earth is large as we continue outwards from the inner solar system.

As we bring the outer planets to the table, Earth shrinks down to a very small size equal to about 3X the size of the Great Red Spot on Jupiter. The rings of Saturn aren't included leaving only the comparative sizes of the spheres themselves. But now we have some idea of what the true relative sizes of the 9 planets of our solar system are when brought close to each other.

Click on Read More for the rest of this story, and by-the-way, to begin feeling very, very, VERY small indeed!

As we bring our own Sun into the mix, we are still only just beginning to see how really small we are, compared to our Earth, our other planetary bodies, and our own star which, by-the-way is classified as a common yellow dwarf star.
How small Earth appears next to the Sun, and now even Jupiter looks small as well. But we are still within the confines of our own solar system, and our home is about to disappear compared with some neighboring stars.

To get a feel of why our star is called a "dwarf" we'll journey beyond our solar system to some familiar nearby stars, such as Sirius, the brightest true star in our night skies. (Notice that Jupiter at this scale is represented as a single pixel in image 4 below!) Next out is Pollux, one of the twin stars of Gemini along with Castor (which is not shown), and then Arcturus. Sirius is a mere 8.6 light-years away, while Pollux is about 33 ly away, and Arcturus is 36 ly away. Yet even Arcturus begins to seem small when we compare it to the stars in the 5th image. In this image, even our Sun is reduced to the size of a single pixel! Two of the most widely known stars of the constellation Orion are used here to continue our scale model, these being Rigel, a so-called blue giant star representing the left knee of Orion, and Betelgeuse, a red-giant demarkating the right shoulder of the Mighty Hunter. Betelgeuse lies 427 ly away, and Rigel is 772 ly away. That says a lot about their individual luminosities, too.

Interestingly enough, the largest star in our scale model here is Antares, (1,000 light-years away)a reddish-orange truly giant star representing the heart of the scorpion.
Notice that now our own Sun is just barely visible compared to the other stars of this scale model of neighboring stars. NOW, we can finally start feeling small, right?
In Greek mythology, Orion was the favorite son of Zeus, who had imparted immortality to Orion except for one weak spot at his heel. Orion was overly boastful of his hunting prowess and didn't have a lot of fans. While hunting one day, Orion came upon a scorpion, which happened to sting Orion on his heel exactly where his weakness was, and Orion died from the encounter. Zeus was so distraught from this that he commanded that Orion be lifted up into the night sky as the brightest constellation in the sky, and to help assure that Orion would never be threatened by the scorpion again, Zeus commanded that the scorpion also be taken up on the exact opposite side of the sky from his favorite son.

Big stars burn their supply of hydrogen fuel very quickly and die out quickly. Those like our Sun are conservative in their consumption of hydrogen and are typically around for 10 billion years or so. Thankfully we live near a star that is only about 1/2 way through its hydrogen supply and its expected life-cycle.
Notice that there are several stars represented here which all resemble each other in color. Color, in astronomy, is an indicator of temperature. All of the reddish-orange stars depicted are of the same "class" of stars, called red giants. Red giant stars are at the end of their life cycles, when the original fuel they started with has been used up completely and the internal structure of the star begins to expand, while also becoming very thin and tenuous. Red giants are not long for this universe, and they are ONLY large due to the current stage of their life cycle.

For all that this scale model does to help us garner some early idea of how really small and insignificant we Earthlings are in the grand scheme of the universe, there are even bigger stars out there. Giants that are young in their life cycles, having simply been born recently, and huge from the beginning.

Astronomers believe that there are stars more than 100 times the size of our Sun, and possibly as much as 200 times larger or more. However, these gigantic stars are few and relatively far in-between one another. Out of the estimated 200 billion stars in our own Milky Way Galaxy, there may only be a hundred thousand or so of them, perhaps even fewer.

But stars the size of our Sun or smaller are many times more numerous making up perhaps 80% to 85% of the population in the galaxy. Astronomers have also only recently begun to locate these smallest stars, the ones that may help increase our galay's population to upwards of 300 to 400 billion when we have a better idea of just how numerous they might actually be. These are the dimmest of all stars, the red-dwarfs, only 1/10th the size of our Sol or perhaps slightly less.

The hunt goes on. Its my hope that there may be some young astronomer from our area who will go on to someday help fill-in the gaps in our knowledge of the life-cycle of stars in the universe. I can say with absolute confidence that the universe they inherit from their predecessors will be more vast, and infinitely more complex and wonderful than we have yet imagined. I wish I were in a position to be one of those young astronomers, but I'll happily retain my status as just someone who occasionally fosters the interest that may someday lead others into that field.
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