Hubble was mind-blowing, but we're just getting started

People observe the night sky with a telescope  (AFP)
People observe the night sky with a telescope  (AFP)

Summary

  • It’s been an eventful, fruitful three decades for the Hubble Space Telescope. But as with all tools, newer and more sophisticated ones eventually come along

If I go out on my terrace at about midnight tonight and look northeast, it’s likely I will see Ursa Major, or the Great Bear—the familiar ‘question mark’ constellation. Its seven stars are just bright enough to shine through the city’s ambient light that drowns out most fainter stars. But if you live in a smaller town, or in a remote village, you may see the few more stars that make up Ursa Major. You may even realize it isn’t really a question mark, that it does sort of suggest a bear.

Though in that remote village, you may not pay much attention to Ursa Major itself. Because if you’re lucky, the night sky will be awash with stars. That’s the reality of the universe—stars every which way you look. You may even see the Milky Way, our spectacular home galaxy, arcing gracefully across the sky like a delicate scarf. If you do, let that be a reminder that a lot of those “stars" you see are actually themselves galaxies, made up of billions upon billions of stars. Only, they are so unimaginably far away that if we do see them, they look like points of light. Like stars.

One night in 1995, astronomers pointed a telescope at Ursa Major. Or actually, they pointed it at a small segment of the sky within the Bear’s outline. To the naked eye, there’s nothing to be seen in that particular little patch. Even on a dark night in that remote village, you might see no more than a few faint dots of light in the vicinity. But this telescope is in a quite different league from the naked eye. In fact, it is in a different league from pretty much every other telescope on Earth.

That’s easy to say, because this telescope isn’t on Earth anyway. It’s in space, orbiting our planet. From there, its ability to look out into the void exceeds any other optical instrument we have on the planet. This is the Hubble Space Telescope (HST), named for the great astronomer Edwin Hubble. The space shuttle Discovery carried the HST into space in 1990, and its astronauts assembled it.

The HST has a mirror 2.4 metres across, which is actually small compared to dozens of Earth-bound telescopes. The largest of those is the Gran Telescopio Canarias, on the Canary Islands, whose mirror is a gigantic 10.4m across. Certainly a larger mirror means more light collected and fainter objects detected, but the HST has the great advantage of location. Up there in space, it is not subject to the vagaries of weather, pollution and atmospheric distortions that telescopes sitting on the planet have to contend with. Since it orbits the Earth, its view of space is not restricted either, like with ground-based telescopes. This is why Nasa says the HST has “a crystal-clear view of the universe."

The result is that in the 31 years since it climbed into orbit, the HST has truly revolutionized both the way we look at the universe and our understanding of it. Through it, astronomers have seen galaxies and stars further away than anything previously observed. It has peered at objects that are several billion light years away, in effect looking back that many years to near the birth of the universe. It has told us the composition of the atmospheres of planets that whirl around other stars. And astronomers have also trained the HST on considerably closer objects, like the planets in our solar system. It once watched a comet slam into Jupiter. It found that Pluto, itself now seen as a quasi-planet, possesses moons.

Besides, it has given us some of the most gorgeous images we have of objects that are out there. Look for its “Pillars of Creation", vast clouds of gas and dust in the Eagle Nebula, some 7,000 light years from us. Or the cluster of young stars—only a few million years young—called R136 in the Tarantula Nebula, about 160,000 light years away. Or its images of Jupiter, taken about two months apart in 2007, showing dramatically how that planet’s cloud cover changes colour and shape.

And there’s what’s come to be known as the Hubble Deep Field: the HST’s Ursa Major adventure of 1995.

HST’s position above Earth’s atmosphere means it is uniquely capable of observing distant galaxies in far greater detail than would be possible from a ground-based telescope. In fact, such observation was one of the primary goals of those who designed HST, and this is what drove the Hubble Deep Field project.

But where do we look for such galaxies? First, it’s important to direct your line of sight away from the plane of the Milky Way, so you’re not looking through our galaxy as you search. Second, there are still Milky Way stars all over the sky. It’s hard to find a part of the sky that doesn’t have at least a few. Third, it is also vital to point the HST at a target that does not get obscured by the Earth itself as the telescope orbits. These and some other criteria narrowed the search to three small patches of sky, all within Ursa Major, and then one of those was selected.

How small was this patch of sky? Hold a cricket ball up at arm’s length: how much of the sky does it cover? Now imagine your arm is 100 metres long: now how much of the sky does the ball cover? That’s about the size of the Hubble Deep Field target—about one twenty-four-millionth of the total area of the sky.

The HST stared at this little scrap of the night sky for a total of 140 hours. What does that mean? Consider that your typical snapshot uses a shutter speed of 1/60th of a second—the time to gather enough light for the image. In December 1995, the HST effectively had its shutter open not for 1/60 second, but for 140 hours. That’s a measure of how dark that part of the sky is, how faint any sources of light there really are.

But when astronomers processed the images from this unblinking Hubble stare, they found about 3,000 sources of light. About 20 turned out to be nearby stars—indeed, they are all over the sky. But the rest of the 3,000 were galaxies. Some of them were the most distant and thus the youngest galaxies we have ever observed—just 1.5 billion years old. If that still seems ancient to you, remember that it’s been about 14 billion years since the Big Bang and the birth of the universe. So a galaxy that is 1.5 billion years old is young indeed. This is why astronomers say the HST has taught us plenty we did not know about the early universe and how galaxies are formed.

It’s been an eventful, fruitful three decades for the Hubble Space Telescope. But as with all tools, newer and more sophisticated ones eventually come along. That’s where we are with the HST. A new space telescope has a mirror nearly three times larger than the HST’s. You can imagine how much more powerful it will be at gathering wisps of light. It will ride into space later this month, and an astronomer at the University of Texas explained, “I think we’ll be in for some surprises."

Another astronomer, at the University of Exeter, was less guarded: “James Webb will blow the lid off everything."

Next time: The James Webb Space Telescope.

Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. His Twitter handle is @DeathEndsFun

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