Does The Expansion Of The Universe Affect The Constellations?

Considering the Universe’s expansion, has the distance of the stars like the Orion’s belt ones changed in a noticeable magnitude for our naked eyes along our lives? Or does the fact that they are in our galaxy maintain them at the same distance always?
Alnitak, Alnilam, and Mintaka, are the bright bluish stars from east to west (left to right) along the diagonal in this gorgeous cosmic vista. Otherwise known as the Belt of Orion, these three blue supergiant stars are hotter and much more massive than the Sun. They lie about 1,000 light-years away. Image credit: wikimedia user Astrowicht, CC BY-SA 3.0

Alnitak, Alnilam, and Mintaka, are the bright bluish stars from east to west (left to right) along the diagonal in this gorgeous cosmic vista. Otherwise known as the Belt of Orion, these three blue supergiant stars are hotter and much more massive than the Sun. They lie about 1,000 light-years away. Image credit: wikimedia user Astrowicht, CC BY-SA 3.0

Nothing in the universe is completely still, but our Universe behaves much more like your second option than the first one.

You’re absolutely right that things within the galaxy are not expanding along with the Universe at large, and this is because everything within the galaxy is gravitationally attached to the galaxy as a whole, and is not so easily extracted. At the moment, the force which pushes the Universe to accelerate its expansion (the infamously poorly named Dark Energy) is weaker than the attractive force of gravity, which pulls objects together. This is fortunate for us, because it means our galaxy is not being sheared apart by the expansion of the Universe.

The relative strength of gravity in our Universe ensures that anything that’s gravitationally tied to another object is not doing any drifting away from its companion due to the expansion of the Universe. This holds for any set of objects which are ruled by gravity —  the stars within a galaxy to the galaxy, or two stars to each other, or two galaxies to each other.

Now, this is not to say that these objects aren’t moving relative to each other — just that this motion is not driven by the Universe’s expansion. It’s driven entirely by gravity. All the stars in our galaxy are following their own orbits around the center of our galaxy, and these orbits are not always perfect circles, so any two stars may find themselves at slightly different distances if you watch long enough.

This image, the first to be released publicly from VISTA, the world’s largest survey telescope, shows the spectacular star-forming region known as the Flame Nebula, or NGC 2024, in the constellation of Orion (the Hunter) and its surroundings. In views of this evocative object in visible light the core of the nebula is completely hidden behind obscuring dust, but in this VISTA view, taken in infrared light, the cluster of very young stars at the object’s heart is revealed. The wide-field VISTA view also includes the glow of the reflection nebula NGC 2023, just below centre, and the ghostly outline of the Horsehead Nebula (Barnard 33) towards the lower right. The bright bluish star towards the right is one of the three bright stars forming the Belt of Orion. The image was created from VISTA images taken through J, H and Ks filters in the near-infrared part of the spectrum. The image shows about half the area of the full VISTA field and is about 40 x 50 arcminutes in extent. The total exposure time was 14 minutes. Image credit: ESO/J. Emerson/VISTA. Acknowledgment: Cambridge Astronomical Survey Unit

This image, the first to be released publicly from VISTA, the world’s largest survey telescope, shows the spectacular star-forming region known as the Flame Nebula, or NGC 2024, in the constellation of Orion (the Hunter) and its surroundings. In views of this evocative object in visible light the core of the nebula is completely hidden behind obscuring dust, but in this VISTA view, taken in infrared light, the cluster of very young stars at the object’s heart is revealed. The wide-field VISTA view also includes the glow of the reflection nebula NGC 2023, just below centre, and the ghostly outline of the Horsehead Nebula (Barnard 33) towards the lower right. The bright bluish star towards the right is one of the three bright stars forming the Belt of Orion. The image was created from VISTA images taken through J, H and Ks filters in the near-infrared part of the spectrum. The image shows about half the area of the full VISTA field and is about 40 x 50 arcminutes in extent. The total exposure time was 14 minutes. Image credit: ESO/J. Emerson/VISTA. Acknowledgment: Cambridge Astronomical Survey Unit

The stars in Orion’s belt are no exception. They come with the wonderful names of Alnilam, Alnitak and Mintaka, and sit relatively close to us within our galaxy. For some scale, our Sun is about 30,000 light years from the center of our galaxy. These three stars, by contrast, are 1,340 light years, 817 light years, and 916 light years distant, respectively. And these stars are moving, relative to us; Alnilam is moving directly away from us at about 25.9 kilometers every second (it’s also moving sideways, but it’s the traveling away from us part which might be able to make the star fainter). This translates to 58,000 mph, which in astronomical terms is very, very slow. The other two stars are moving even slower — around 18.5 kilometers per second (~41,000 mph).

Considering that a light year is about 5.8 trillion miles (that’s a five, and then 12 zeros), you’re going to have to watch these stars for a really long time for them to make it even a single light year more distant from us. By my calculation, Alnilam, our fastest-moving star, will need about 11,450 years to travel the 5.8 trillion miles in a light year. That star is already sitting at 1,340 light years from us, so an additional light year changes the distance to that star by less than a tenth of a percent — our eyes won’t notice this change, even if we had the 11 thousand years to wait.

Read the full article on Forbes!

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How Related Are The Stars In A Constellation?

 

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What constellations would have been visible at the Nativity?

Whole-sky star chart depicting the Milky Way in blue. Image Credit: Wikimedia Commons user Tfr000, CC BY-SA 3.0

Whole-sky star chart depicting the Milky Way in blue. Image Credit: Wikimedia Commons user Tfr000, CC BY-SA 3.0

Originally posted on Forbes!

If you go outside tonight or tomorrow for the Christmas celebrations, and you’re in the Northern Hemisphere, you’ll see the familiar constellations of winter. Orion is one of the more prominent constellations, visible to the south, with three of the brightest stars in the sky. Just underneath Orion’s left knee is Sirius, the brightest star in the northern hemisphere. Above Orion shines Taurus, and above Taurus, the small cluster of stars known as the Pleiades.

To the north we would see the Big Dipper, which is technically Ursa Major (the large bear), and by following the line drawn by the two stars at the end of the dipper, we can trace our way through the sky to the North Star, Polaris.

The North Star is remarkable because it doesn’t move over the course of the night; this is because our planet’s axis of rotation points pretty directly at that star in our sky. However, this axis isn’t stable over time. There’s a 24,000 year cycle, where our planets rotation gradually shifts the direction it points in the night sky. 2000 years ago, Polaris would have been noticeably further away from the axis of rotation, which means it would spin in a little arc over the course of the night, and change its position relative to North over the course of the year. Possibly a better proxy for a North Star at the time would have been Kochab, a star at the other end of the Little Dipper, but nothing would have been as accurate a pointer as Polaris currently is.

There have been some suggestions by biblical scholars that the historical Nativity might have happened earlier in the year, perhaps sometime between spring and late summer. If that’s the case, then the constellations you might see would be a little different. Anything visible in winter between the north star and the horizon would still be visible, but if you’re in summer, your brightest constellations are overhead; Lyra, with Vega as its brightest star, Cygnus, with Deneb as its brightest star, and Aquila, with Altair as its brightest star. Between the three bright stars, they trace out the Summer Triangle, directly overhead.

Tomorrow night (Christmas night), there is a full moon. If it’s clear where you live, head outside and look up.


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