Celestial Tour: Long Ago and Far Away

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 We see distant galaxies not as they are today, but as they were many millions and even billions of years ago. 

Produced by the Space Telescope Science Institute’s Office of Public Outreach in collaboration with NASA’s Universe of Learning partners: Caltech/IPAC, Center for Astrophysics | Harvard & Smithsonian, NASA Jet Propulsion Laboratory, and Sonoma State University 


 Heavenly bodies glow in the night sky 

Text, Looking deep into the universe, we see a dazzling array of celestial bodies. 

We see them at vast distances both in space and in time 

Text, Long Ago and Far Away 

detailed photograph of the moon 

Text, The moon is Earth's nearest neighbor in space and the farthest humans have travelled into the universe 

Photo of astronaut planting American flag on the moon 

Moon and Earth 

Text, This image from the Voyager spacecraft is the first ever to capture both Earth and Moon in a single frame 

They are celestial companions in a seemingly dark and empty cosmos. 

However, they are not as close as this picture might suggest. 

A tiny earth to the left edge, tinier moon to the right, separated by a vast, black expanse. 

The true scale between Earth and the Moon looks more like this: 

Earth's diameter is roughly four times the Moon's diameter. 

Their separation is just over 30 times the Earth's diameter 

30 Earths appear between the Earth and Moon 

This distance is approximately 230,000 miles. 

squiggly line moves from the Earth toward the moon 

Even at the speed of light, it takes about one and a quarter seconds to travel from Earth to the Moon. 

This fact explains the two-and-a-half second delay in radio conversations between Mission Control and Apollo astronauts on the Moon 

Picture of Mission Control on the left, astronaut on the moon on the right 

Astronomers use light-travel time as a measure of distance. In these units, the Moon is 1.28 light-seconds away from Earth. 

Light travel time has proven to be the most useful distance measure of the universe. 

For example, the moon's orbit stretches across two and a half light-seconds of space, while the Sun itself is more than four light-seconds in diameter, and is located about 500 light-seconds away, which is more than 8 light-minutes 

Circle with "Earth" written in the center, a small dot labeled "moon" on the perimeter, and the sun illustrated as a gigantic yellow ball. 

Earth shown as a tiny dot to the left of the screen, Sun a slightly larger yellow dot to the right 

Sun shown through a powerful telescope 

because of this light-travel time, we see eruptions on the sun more than eight minutes after they happen 

illustration of dot surrounded by widening circles labeled with names of planets 

The size of the solar system, several billion miles across, is measured in light-hours 

Telescopic photo of Neptune 

It takes at least four hours for light from Neptune to reach Earth 

Much farther away are the stars 

Telescopic photo of many, many stars in the night sky 

Alpha Centauri and Proxima Centauri are circled on the photo 

Alpha Centauri is the nearest bright start, but slightly closer is the faint dwarf star Proxima Cenauri 

Even from these nearest of stars, the light-travel time is over 4 years. 

Telescopic photo of the Milky Way 

Our Milky Way Galaxy is a giant pinwheel of some hundred billion stars 

The Sun is located about 24 thousand light-years from the galaxy's center. 

A star within our galaxy might explode today, and we wouldn't know about it until many thousands of years later. 

The nearest large neighbor to our Milky Way is the Andromeda Galaxy 

The light we see today was emitted by stars in Andromeda about two and a half million years ago, during the time of our early human ancestors roamed Africa 

Telescopic map of known galaxies 

Galaxies can gather together in vast clusters 

This grouping is at the center of the Virgo Cluster of galaxies 

The entire Virgo Cluster contains many thousands of galaxies. It is the largest nearby galaxy cluster. 

Located over 60 million light-years away, these galaxies appear to us as they were at the end of the age of the dinosaurs on Earth 

Such long light-travel times mean that a telescope looking out into space is also looking back in time 

We see distant galaxies not as they are today, but as they were many millions and even billions of years ago. 

To look deep into the universe, astronomers must take long exposures, collecting light for many hours or days. 

Deeper exposures reveal fainter features and greater detail. Here is a series of images of the Andromeda Galaxy exposed for 1, 5, 30, and 45 minutes. 

image on the right side of the screen slowly blooms from a blurry white smudge into a large, swirling galaxy 

Photo taken with the Hubble telescope, showing celestial bodies as orange, yellow, blue, red, and white lights 

The deepest visible-light exposure ever taken is the Hubble Ultra Deep Field. This observation lasted more than eleven days. Some 10 thousand galaxies can be seen in this image. 

All these galaxies are found in a tiny region of the sky. about the same apparent size as the Sea of Serenity on the Moon as seen from Earth 

galaxies in such deep images are typically many billions of light-years away. 

What appears to be a 2D image is actually a 3D collection of galaxies 

Flat image rotates and spreads out showing a timeline of 1, 3, 5, and 10 billion light years 

Nearby spiral galaxies, seen in the recent past, show well developed pinwheel shapes 

More distant galaxies, seen as they were several billion years ago, are smaller and have incomplete spiral structure 

The most distant galaxies, seen as they were many billions of years ago, are tiny and just beginning to develop their spiral shapes. 

Assembling them together, one can visually examine the history of galaxy evolution. Looking billions of light-years out into space, we see billions of years back in time. 

photos of galaxies labeled present to 3 billion years ago, 3 to 7 billion years ago, and 7 to 10 billion years ago 

Humans have traveled a mere one light-second into the universe. But even that tiny distance grants us a whole new perspective 

Observations from the Hubble Space Telescope can take us across billions of light-years. 

A journey through the Hubble Ultra Deep Field, across both space and time, takes us to galaxies long ago and far away 

Stars and galaxies float by 

Produced by: Space Telescope Science Institute, Office of Public Outreach. Photo credits. 

For more information, visit Hubble Site: http://hubblesite.org/newscenter/archive/releases/2004/07/. The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, Inc., for NASA under contract with NASA's Goddard Space Flight Center, Greenbelt, Maryland.