Peering into the Past

Peering into the Past

Dan Brubaker and a mishmash of know-nothings convene at the Dr Ralph L Buice, Jr Observatory, Atlanta

A low, steady rumble vibrates through the air
and under my feet. It is the sound of a garage door closing, only this is no garage. The single curved wall that surrounds the room remains remarkably still. It
is the mechanical domed ceiling perched overhead
that is causing the ruckus, that is rotating with careful precision – like a combination lock under the hand of a gigantic and expert safecracker.

The dome comes to rest and returns our attention to
its rightful place, stationed in the centre of the space: a telescope shaped like a monumental hairdryer. It stares through a rectangular opening in the domed ceiling to feast on a tiny, yet endless, sliver of night sky.

This is why I am here – why this mishmash group of astronomical amateurs and know-nothings has convened on a clear Thursday night. We are at the Dr Ralph L Buice, Jr Observatory, part of the Fernbank Science Center in Atlanta, Georgia.

One of our cosmic tour guides, a volunteer at the science centre, wheels a metal stairway into position under the telescope and explains what we’re about to see: the Orion Nebula, a collection of four stars surrounded by a diffuse cloud of dust. It is within interstellar gas clouds like this one that stars are formed; nebulas are the universe’s stellar nurseries.

My turn arrives, and I take a few steps up the stairway. 
As I twist my neck to peer through the eyepiece, a second volunteer tells us that the Orion Nebula is over 1,300 light-years away. That means that what I’m seeing, this amorphous haze of periwinkle and magenta punctuated with four stellar pinpoints, reflects how the Orion Nebula looked around the start of the 8th century. The light hitting my eyes has been travelling since the time of the Maya.

I step down from the stairs, offering the view to the
 next person in line. Then, standing behind the telescope,
 I try to follow its gaze to see the Orion Nebula without magnification. A volunteer points me in the right direction, shining a laser toward the nebula, and I can just barely make out a hint of the haze.

As well as magnifying, the telescope catches starlight
in 3-foot-wide mirrors, making that hint of haze appear brighter. And by passing the starlight captured by a telescope through a spectrograph, astronomers can generate a spectrum. “If you shine sunlight through a prism it makes a rainbow,” says April Whitt, the astronomy instructor at the Fernbank Science Center. “You can do the same thing for a star. It doesn’t make a very pretty rainbow; it’s just black and white lines. But it’s a fingerprint.”

The spectrum separates the star’s light out by wavelength, and by inspecting it we can glean a whole suite of information. We can calculate the star’s temperature by noting which wavelengths along the spectrum appear brightest. Vertical black absorption bands at reliable wavelengths indicate which elements are present within the star’s gaseous surface. The width of these bands hint at the star’s size. And, taken together, a star’s temperature and size will determine its brightness.

“You can have a very low temperature star that is enormous. And, as a result, it has a big surface area. So it’s really bright,” says Chris De Pree, a professor of astronomy at Agnes Scott College in Decatur, Georgia. “Betelgeuse is a low temperature star, but if you put it in the solar system it would go out to the orbit of Jupiter. It’s just absolutely enormous. So that kind of surface area means it’s just really intrinsically bright.”

Brightness, temperature, composition – all of this spectral information allows astronomers to classify a star. O-type stars have temperatures over 25,000 Kelvin, and their spectra include hydrogen and helium bands. F-type stars are cooler (6000 to 7500 Kelvin); their spectra include bands that correspond to hydrogen, calcium, and iron. But knowing, by virtue of its type, the actual brightness of a star – especially a faintly glimmering one – is particularly useful.

“Each type of star is kind of like a different wattage of light bulb. So if you know how bright something appears and you use its spectrum to figure out how bright it actually is,” says De Pree, “you can figure out the distance.” A type 1 supernova in our galaxy, for example, will appear much brighter than a type 1 supernova in the neighbouring Andromeda Galaxy. The fainter the star, the farther the distance, the further back in time.

Though the immenseness of these distances is foreign
to me, a background in anthropology has trained me to think in long stretches of time. And I can now see parallels between the history of the universe that hangs above us and the history of man buried beneath our feet. Archaeology discovers the past by digging down, looking under our planet’s crust, uncovering fossils trapped between stony sediments or swallowed up in amber. Astronomy probes upward, looking outside of our planet into the surrounding universe, peering past layers of cosmic gas at light caught up in the magnitude of space. The unaided eye can see as far as the Andromeda Galaxy, some 2.5 million light-years away. To look at Andromeda is to receive photons that left their stars before Homo erectus left Africa.

With telescopes, the limit to how far into the past we can see has less to do with distance and more to do with the existence of stars to set our sights on. Astronomical estimates date the Big Bang to between 13.5 and 14 billion years ago. But stars themselves take millions of years to form.

Astronomers have observed the radiation from the universe’s first stars. Only, the radiation from a source that far back doesn’t reach us as visible light because of how
the universe is expanding. The result of this expansion demonstrates the Doppler effect: like the drop in pitch you notice when a car blares its horn while whizzing past you, the waves emitted from receding stars have lower frequencies when they reach us. Since the speed of light
in space is constant, a lower frequency means a greater wavelength. So for astronomers looking at stellar spectra, the Doppler effect presents itself as redshift – those black absorption bands shift toward the red side of the visible spectrum. For stars at sufficient distances, a greater redshift corresponds to a faster rate of expansion away from us.

Those first stars are receding so rapidly that the waves they radiate are stretched across the electromagnetic spectrum, redshifted well beyond the range of visible
light, beyond the infrared – even surpassing microwave frequencies. In order to get a clear image of stellar radiation that is almost as old as the universe itself, we must rely on radio telescopes.

Squeezing past the crowd to exit the dark domed room, I spill out onto the observatory roof and drift through the still open air. Compared to the photons reaching us via radio waves after billion-year-long marathons, the journey between nearby constellations and our eyes would hardly register as a sprint. Even so, as I scan the night sky, I find it hard to appreciate the time in transit. I’m certain my pupils are fully dilated, trying to capture light from stars dominant and dim. I can feel my mind stretching too, trying to comprehend the images it’s receiving.

“As overwhelming as it is and as small as it can make one feel, it still makes me marvel at the power of the human brain,” says De Pree. We may be small, but our collective knowledge of the universe is expansive enough to set my mind spinning.

The observatory dome rumbles back to life, the crowd inside shifting its focus, in time with the dome’s rotation, to another point in the universe’s past. With a weary head and heavy eyelids, I must admit there is a small part of me looking forward to morning when – after the Earth does some rotating of its own – I won’t be able to see the stars for the sun, and I can again absorb the easy details of the day, knowing full well that the past will present itself again come night.

Dan Brubaker is a Masters student in Science Writing at Johns Hopkins University, Twitter: @DanBrubaker7. Artwork by Catherine Prowse