Looking Into Space, When Do We Start Looking Into The Past?

While observing an astronomical event at a far away distance from Earth, can we consider the events captured by our strongest telescope happening at an earlier time (past event) being captured by the devices (due to the large distance from Earth) or nearly real-time event (with time in reference to that on Earth)?
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Originally posted on Forbes!

It depends very much on how far away you’re looking! Most things out there could be considered “a far away distance”, even when we’re dealing with objects within our own solar system, but the times involved to travel between Mars and the Earth are much closer to nearly real-time than they are if you start venturing further afield.

Fundamentally, information can only travel through the Universe at the speed of light, and the larger your distances get, the longer it takes light to cross those distances. For anything happening on the Earth, this is not usually much of an impediment, because the distances involved in circling the Earth are not so great. To get from the surface of the Earth to the ISS (a distance of 408,000 meters), light, which travels at 299,792,458 meters every second, is only going to spend about a thousandth of a second (0.0013 seconds) in transit. Events on the ISS can therefore be considered pretty much real time, even though there is a measurable communications lag.

If you go further afield, but still within our solar system, light takes about 1.25 seconds to get to the Moon (so a two and a half second round trip), eight minutes to get from the Sun to the Earth, or about twelve and a half minutes to get to Mars. This all starts to build up to a more considerable time delay, but these are manageable delays - if I got an email response from someone I was writing to in less than 24 minutes I’d think that was pretty rapid.

Hurtling through space at 31,000 miles per hour in this artist's rendering, the New Horizons spacecraft began 21 and a half hours of radio silence as it prepared to collect data for the flyby of Pluto. Image Credit: NASA/APL/SwRI

Hurtling through space at 31,000 miles per hour in this artist's rendering, the New Horizons spacecraft began 21 and a half hours of radio silence as it prepared to collect data for the flyby of Pluto. Image Credit: NASA/APL/SwRI

Once you try talking to the outer solar system, the time delays get a little more significant. The light travel delay to New Horizons when it was swinging past Pluto was about four and a half hours, so to ping New Horizons and hear back instantaneously from the craft, you’d be waiting about nine hours. Somewhere around this kind of time delay, we might start to classify things as happening “in the past”, but this is still a time delay on functional human timescales. Nine hour delays are sending an email to someone and hearing back in the morning. Not so convenient, especially if something complicated is happening in that time, but also not the worst.

It’s when we start looking beyond our solar system and into the Milky Way as a whole, or towards other galaxies that the time delay, which has just been scaling up with the distances involved, gets a little more outrageous. To get information from the center of our own galaxy out to Earth, you have to wait over 26 thousand years. That is no longer a length of time I can wait for an email reply. Information that reaches the Earth from the center of our galaxy is as up to date as it can be, but it’s reporting on changes that happened 26,000 years prior. The changes we see, therefore, are happening at whatever speed we see them happening, but with a time-lag. If we teleported there, it’d be old news.

This image, captured by the NASA/ESA Hubble Space Telescope, shows what happens when two galaxies become one. The twisted cosmic knot seen here is NGC 2623 — or Arp 243 — and is located about 250 million light-years away in the constellation of Cancer (The Crab). Image credit: ESA/Hubble & NASA

This image, captured by the NASA/ESA Hubble Space Telescope, shows what happens when two galaxies become one. The twisted cosmic knot seen here is NGC 2623 — or Arp 243 — and is located about 250 million light-years away in the constellation of Cancer (The Crab). Image credit: ESA/Hubble & NASA

You can imagine that the further out we go, the bigger this problem gets. So, scrolling outwards, the next big thing is Andromeda, which is so far from us that light has been stretching towards us from those stars for 2.5 million years. I think by most standards, this would be considered observing the past, and yet it’s the closest (and therefore informationally least out of date) galaxy we can look at! Most of the rest of the galaxies in the Universe are much further away, and therefore any changes that happen within them are going to be reported to us by our cosmic messenger in light many millions or billions of years later. The one above is 100 times further away than Andromeda, so news from that galaxy will take 100 times longer to reach us.

Where exactly you feel you should put the boundary between “pretty close to real-time” and “definitely looking at the past” is a bit of an arbitrary, fuzzy boundary. If you want to use “how long would you wait for an email reply” as your metric (as I have here), then your boundary is somewhere within the confines of the solar system. But no matter what you want to put down, there comes a point where we are definitely looking into the past, and certainly by the time we’re looking at other galaxies, we’ve reached it.

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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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