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Light As A Cosmic Time Machine by Andrew Fraknoi

"Everything we see in the sky belongs to the past." Timothy Ferris, Seeing in the Dark (p. 116)

The universe tells us its story mainly through light and other wavelengths of electromagnetic radiation. We learn about the planets, stars, and galaxies by their light—visible light, and also shorter-wavelength ultraviolet and longer-wavelength infrared light, invisible to the eye but detectable by certain telescopes on Earth and in space—and by the still longer waves of radio energy that they send us. These waves do not arrive instantaneously. Although they travel at the fastest possible speed (the speed of light,) they take a while to get here. The universe is big, so the news is delayed by the vast gulfs of space it has to cross to reach us. Light covers 186,000 miles EVERY SECOND (kids, please don't try traveling this fast without adult supervision!!!) In metric units, that's about 300,000 kilometers per second.

How long does light take to reach us from familiar objects? Let's take a quick tour of the solar system, asking at each place how long its light takes to reach us here on Earth.

The Moon and the Sun

Moon
The Moon, photographed with a digital camera through a telescope by Timothy Ferris (from Seeing in the Dark).

The closest object to us is the Moon. Its average distance is about 240,000 miles, so light from the Moon takes (240,000 miles divided by 186.000) 1 and 1/3 seconds to get from the Moon to Earth. When astronauts orbited the Moon and later walked on its surface in the 1960's, television viewers noticed that they were slow to answer questions transmitted from Earth. That was because it took 1.3 seconds for the question to travel to the Moon, and another 1.3 seconds for the answer to get back to Earth. Those 2.6 seconds were exactly the round-trip travel time for radio waves between the Earth and the Moon.

The Sun is 93 million miles away, so sunlight takes 8 and 1/3 minutes to get to us. Not much changes about the Sun in so short a time, but it still means that when you look at the Sun, you see it as it was 8 minutes ago. PhotoPhoto of the Sun in hydrogen-alpha light.

The Planets

Jupiter
Jupiter, photographed with an HDTV camera through a backyard telescope by Kenneth Crawford and Michael A. Mayda (from Seeing in the Dark).

The giant planet Jupiter, whose large moons Galileo discovered with his "trouble-making" telescope, is more than 5 times farther from the Sun than the Earth is. We see a planet like Jupiter because its light—which like the other planets and the Moon originates on the Sun—takes about 43 minutes to reach Jupiter. The return trip to the Earth can take from 35 to 52 minutes, depending on whether we are on the same side of the Sun as Jupiter or on the other side.

Little Pluto is so small and remote it was not discovered until 1930, orbits 40 times farther from the Sun than we do. Light from the Sun takes about 5 and 1/2 hours to reach it and roughly the same time to return to Earth. By the time the light reaches us, it has spread out so much that the planet looks very dim, and requires a good telescope to spot. PhotoPhoto of Pluto and its moon Charon, as seen with the Hubble Space Telescope in 1994.

Beyond the Solar System

Moving beyond the Solar System, our scale of distances and travel times needs to change. Now light will require years, not hours, to make its way to us. The star that is nearest to the Sun happens to be part of a system of three stars. (Unlike the Sun, which is a loner, many stars are found in groups of two, three, four or more.) The brightest star in our neighbor system is called Alpha Centauri (pronounced Al' fa Sen' to ree), and it is a virtual twin of the Sun. Light from Alpha Centauri takes more than 4 years to reach the Sun. (Astronomers use a special term for this way of measuring distance—they say the star is 4 light years away.)

Albireo
The double star Albireo, photographed through a telescope by Timothy Ferris (from Seeing in the Dark), is 385 light years from Earth, so the light we see from it tonight was emitted in the seventeenth century.

The brightest star in our skies is the "dog star", Sirius (pronounced Sea' ree us). It's the primary star in the constellation of the big dog, Canis Major. Sirius is roughly 9 light years away. Think of what you were doing 9 years ago. That's when the light we see from Sirius tonight first began its journey to us. Not far from Sirius in the sky is the bright star Betelgeuse (pronounced Beetle' juice). It is so far that its light takes 430 years to reach us. Light that we see tonight from Betelgeuse left it in the late 1500's.

In the same part constellation, Orion, as Betelgeuse but even farther away is the Orion Nebula, a place where we see new stars forming. Its distance is 1500 light years, meaning that the light we see from it left more than a thousand years before the invention of the telescope.

Orion Nebula
The Orion Nebula, a glowing cloud of gas and dust, where we see new stars forming, by Rob Gendler.

The farther away an object in space lies, the longer it takes its light to get to us and the older that light is when it reaches Earth. As we look deeper and deeper into the Milky Way Galaxy (the island of stars in which we live), we are looking deeper into the past. Light can take tens of thousands of years or more to reach us from distant parts of our galaxy, which is roughly 100,000 light years wide.

Other Galaxies

Once we move outside the galaxy, we encounter even larger spaces and longer light travel times. One of the great scientific ideas of the 20th century astronomy was the discovery that there are other galaxies out there—stretching as far as our great telescopes can see. Billions of other islands of stars are scattered through the great dark ocean of space.

Andromeda Galaxy
The Andromeda galaxy, by Rob Gendler.

The nearest large galaxy to the Milky Way is the Andromeda Galaxy. Sometimes astronomers call it M31, by its number in the famous Messier catalog of fuzzy celestial objects. The Andromeda (pronounced An drah' mid a) Galaxy lies about 2 1/2 million light years from Earth. The light we see from it tonight left it more than 2 million years ago, when our species was just beginning to establish its fragile foothold on planet Earth.

In this sense, astronomy is mostly ancient history: The farther away objects are, the older the story they have to tell us. Young people, raised on CNN, the Web, and "instant messaging" may at first bridle at the thought that the most recent information we can get from a neighbor galaxy might be 2 million years old. But for astronomers, this delay in the arrival of light is one of the universe's greatest gifts.

After all, one of the fundamental tasks of astronomy is to fill in the history of the universe—from the Big Bang to the moment you are reading this paragraph. Astronomers might not be able to undertake such a task if the information from the universe were limited to current events. But the universe is a time machine. Looking at more distant objects, we learn about more ancient times and phenomena. Large telescopes allow us to look billions of years into the past and to reconstruct the story of the cosmos eon by eon.

Lookback Times for Some Astronomical Objects
Object Time for the Light to Reach Us
The Moon 1 1/3 sec
The Sun 8 minutes
Jupiter 35 to 52 minutes
Pluto 5 1/2 hours (on average)
Alpha Centauri (nearest star system) 4.3 years
Sirius (brightest star in our sky) 9 years
Betelgeuse (bright star) 430 years
Orion Nebula 1500 years
Andromeda Galaxy 2.5 million years

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