## How Far Back Are We Looking in Time?

When we look out into space, we’re also looking back into time. Just how far back can we see?

The Universe is a magic time window, allowing us to peer into the past. The further out we look, the further back in time we see. Despite our brains telling us things we see happen at the instant we view them, light moves at a mere 300,000 kilometers per second, which makes for a really weird time delay at great distances.

Let’s say that you’re talking with a friend who’s about a meter away. The light from your friend’s face took about 3.336 nanoseconds to reach you. You’re always seeing your loved ones 3.336 nanoseconds into the past. When you look around you, you’re not seeing the world as it is, you’re seeing the world as it was, a fraction of a second ago. And the further things are, the further back in time you’re looking.

The distance to the Moon is, on average, about 384,000 km. Light takes about 1.28 seconds to get from the Moon to the Earth. If there was a large explosion on the Moon of a secret Nazi base, you wouldn’t see it for just over a second. Even trying to communicate with someone on the Moon would be frustrating as you’d experience a delay each time you talked.

Let’s go with some larger examples. Our Sun is 8 minutes and 20 seconds away at the speed of light. You’re not seeing the Sun as it is, but how it looked more than 8 minutes ago.

On average, Mars is about 14 light minutes away from Earth. When we were watching live coverage of NASA’s Curiosity Rover landing on Mars, it wasn’t live. Curiosity landed minutes earlier, and we had to wait for the radio signals to reach us, since they travel at the speed of light.

When NASA’s New Horizons spacecraft reaches Pluto next year, it’ll be 4.6 light hours away. If we had a telescope strong enough to watch the close encounter, we’d be looking at events that happened 4.6 hours ago.

The closest star, Proxima Centauri, is more than 4.2 light-years away. This means that the Proxima Centurans don’t know who won the last US Election, or that there are going to be new Star Wars movies. They will, however, as of when this video was produced, be watching Toronto make some questionable life choices regarding its mayoral election.

The Eagle Nebula with the famous Pillars of Creation, is 7,000 light-years away. Astronomers believe that a supernova has already gone off in this region, blasting them away. Take a picture with a telescope and you’ll see them, but mostly likely they’ve been gone for thousands of years.

The core of our own Milky Way galaxy is about 25,000 light-years away. When you look at these beautiful pictures of the core of the Milky Way, you’re seeing light that may well have left before humans first settled in North America.

And don’t get me started on Andromeda. That galaxy is more than 2.5 million light-years away. That light left Andromeda before we had Homo Erectus on Earth. There are galaxies out there, where aliens with powerful enough telescopes could be watching dinosaurs roaming the Earth, right now.

Here’s where it gets even more interesting. Some of the brightest objects in the sky are quasars, actively feeding supermassive black holes at the cores of galaxies. The closest is 2.5 billion light years away, but there are many much further out. Earth formed only 4.5 billion years ago, so we can see quasars shining where the light had left before the Earth even formed.

The Cosmic Microwave Background Radiation, the very edge of the observable Universe is about 13.8 billion light-years away. This light left the Universe when it was only a few hundred thousand years old, and only now has finally reached us. What’s even stranger, the place that emitted that radiation is now 46 billion light-years away from us.

So crack out your sonic screwdrivers and enjoy your time machine, Whovians. Your ability to look out into space and peer into the past. Without a finite speed of light, we wouldn’t know as much about the Universe we live in and where we came from. What moment in history do you wish you could watch? Express your answer in the form of a distance in light-years.

## How Far Can You See in the Universe?

When you look into the night sky, you’re seeing tremendous distances away, even with your bare eyeball. But what’s the most distant object you can see with the unaided eye? And what if you get help with a pair of binoculars, a telescope, or even with the Hubble Space Telescope.

Standing at sea level, your head is at an altitude of 2 meters, and the horizon appears to be about 3 miles, or 5 km away. We’re able to see more distant objects if they’re taller, like buildings or mountains, or when we’re higher up in the air. If you get to an altitude of 20 meters, the horizon stretches out to about 11 km. But we can see objects in space which are even more distant with the naked eye. The Moon is 385,000 km away and the Sun is a whopping 150 million km. Visible all the way down here on Earth, the most distant object in the solar system we can see, without a telescope, is Saturn at 1.5 billion km away.

In the very darkest conditions, the human eye can see stars at magnitude 6.5 or greater. Which works about to about 9,000 individual stars. Sirius, the brightest star in the sky, is 8.6 light years. The most distant bright star, Deneb, is about 1500 light years away from Earth. If someone was looking back at us, right now, they could be seeing the election of the 52nd pope, St. Hormidas, in the 6th Century.
There are even a couple of really bright stars in the 8000 light year range, that we might just barely be able to see without a telescope. If a star detonates, we can see it much further away. The famous 1006 supernova was the brightest in history, recorded in China, Japan and the Middle East.

It was a total of 7,200 light years away and was visible in the daytime. There’s even large structures we can see. Outside the galaxy, the Large Magellanic Cloud is 160,000 light years and the Small Magellanic Cloud is almost 200,000 light years away. Unfortunately for us up North, these are only visible from Southern Hemisphere.The most distant thing we can see with our bare eyeballs is Andromeda at 2.6 million light years, which in dark skies looks like a fuzzy blob.

If we cheat and get a little help, say with binoculars – you can see magnitude 10 – fainter stars and galaxies at more than 10 million light-years away. With a telescope you can see much, much further. A regular 8-inch telescope would let you see the brightest quasars, more than 2 billion light years away. Using gravitational lensing the amazing Hubble space telescope can see galaxies, incredibly far out, where the light had left them just hundreds of millions of years after the Big Bang.

If you could see in other wavelengths, you could see different distances. Fortunately for our precious radiation sensitive organs, Gamma and X rays are blocked by our atmosphere. But if you could see in that spectrum, you could see objects exploding billions of light years away. And if you could see in the radio spectrum, you’d be able to see the cosmic microwave background radiation, surrounding us in all directions and marking the edge of the observable universe.

Wouldn’t that be cool? Well, maybe we can… just a little. Turn on your television, some of the static on the screen is this very background radiation, the afterglow of the Big Bang.

What do you think? If you could see far out in the Universe what would you like a close up view of? Tell us in the comments below.

## Proxima Centauri

[/caption]As the nearest star from our Solar System, Proxima Centauri is a prime candidate for future interstellar travel and space colonization missions.

In the meantime, scientists are trying to determine whether this star has super Earths orbiting within its habitable zone. Habitable zones are regions around a star where planets are believed to receive just the right amount of heat. For instance, Earth is within the Sun’s habitable zone.

If we were slightly nearer, say on Venus’ orbit, the heat would have evaporated all our oceans. On the other hand, if we were slightly farther, the temperature would have been too cold to support life.

So far, searches in the neighborhood of Proxima Centauri have revealed nothing. Even companion stars or supermassive planets that may be accompanying the star have not yet been discovered (if they are ever there at all). Although the search continues, some scientists believe Proxima Centauri’s flares can be a big obstacle for life even inside the star’s habitable zone.

Proxima Centauri’s flares are believed to be caused by magnetic activity. When a flare occurs, the brightness of all electromagnetic waves emitted by the star increases. This includes radio waves as well as harmful X-rays. The most common flare stars are red dwarfs, just like Proxima Centauri.

Now, even if Proxima Centauri is the nearest star, it is still 4.2 light years away. That’s about 4 x 10 13 km. The spacecraft that would take the first explorers to that system would have to rely on a virtually unlimited supply of energy. Furthermore, sufficient shielding against cosmic radiation should be in place.

Proxima Centauri is smaller than our Sun with a mass of approximately 0.123 solar masses and a radius of only about 0.145 solar radii. Its interior is believed to be totally dependent on convection when it comes to transferring heat from the core to the exterior.

Discovered in 1915 by Robert Innes, the Director of the Union Observatory in Johannesburg, South Africa, the star was observed to have the same proper motion as Alpha Centauri. Further studies confirmed that it was in fact very close to Alpha Centauri. The current distance between the two is estimated to be about only 0.21 light years.

Here are some articles in Universe Today that talk about Proxima Centauri:

What is the nearest star to the Sun?

How far is the nearest star?

Can’t get enough of stars? Here’s Hubblesite’s News Releases about Stars, and here’s the stars and galaxies homepage..

We have recorded several episodes of Astronomy Cast about stars. Here are two that you might find helpful: Episode 12: Where Do Baby Stars Come From, and Episode 13: Where Do Stars Go When they Die?

Source: Wikipedia

## How Far is a Light Year?

A light year is a standard of measurement used by astronomers to describe huge distances in the Universe. The nearest star is 4.22 light years away. The center of the Milky Way is about 26,000 light years away. But how far is 1 light year? A light year is the distance that light travels in a single year. And light travels fast.

1 lightyear is 9,460,730,472,580.8 kilometers.

Need some other measurements? A light year is 5,878,625,373,183.6 miles. And a light year is 63,241 astronomical units (1 astronomical unit, or AU is the average distance from the Earth to the Sun).

But a light year isn’t the largest measurement tool astronomers have. That’s a parsec. 1 parsec = 3.26156 light years.

We have written several articles about measuring distance for Universe Today. Here’s an article about the speed of light, and an article about a device that makes radio waves go faster than the speed of light.

If you’d like more information on light years, here’s an article about what a light year is and how it’s used, and here’s a cool video that shows you how far a light year is.

We’ve also recorded an episode of Astronomy Cast about different tools for measuring distance in the Universe. Listen here, Episode 10: Measuring Distance in the Universe.