[Music throughout] How can astronomers determine
distances in the far reaches of the universe? A small galaxy
close in looks similar to a large galaxy farther out.
This is real challenge that researchers have found several
solutions for. One method uses something called a
standard candle. A standard candle is a type of object or
event that emits a specific, known amount of light, allowing
scientists to find its distance with a straightforward formula.
This works because light sources appear predictably dimmer
the farther they are from an observer. Since astronomers know
how much light a standard candle gives off, they can determine its distance
by measuring how dim it appears from Earth.
Since only very bright objects or events are visible in the
far reaches of the universe, the options for standard candles are limited.
Some of the best and most reliable are exploding stars,
called supernovae. There are a few different kinds of supernovae,
but the best for standard candles are Type Ia. These supernovae
involve a white dwarf — the leftover core of a dead star — and
one other star in a binary system. Some of the time
it may be a white dwarf and larger “host” star.
Scientists think the white dwarf steadily accumulates material shed by
the host star, gaining mass in the process.
When it reaches a specific tipping point, the white dwarf has gained enough
mass to trigger a runaway reaction at its core
and it explodes spectacularly, sending out an expanding sphere
of super-hot material that glows from the energy of the explosion.
In other cases, scientists think two white dwarf stars may
form the binary. Either the stars finally merging together
triggers the supernova, or it happens as they spiral in
closer and closer, while the more massive of the two pulls material
off its companion in the final few minutes.
Before they merge, it reaches the same mass tipping point and goes
supernova, always releasing a similar amount of energy.
Because white dwarf explosions are all so similar,
the energy and light output of Type Ia supernovae are easy
to standardize. Type Ia supernovae are rare in any one
galaxy, occurring only once every 500 years or so in the
Milky Way. But because there are so many galaxies, astronomers
using current telescopes observe Type Ia supernovae about a
hundred times a year. By comparing the observed brightness with
the intrinsic brightness, astronomers can determine their distances
within 6 percent. The Nancy Grace
Roman Space Telescope, set to launch in the mid-2020s,
will observe large patches of sky repeatedly, increasing
the opportunities to spot these supernovae.
Scientists predict Roman will see as many supernovae in one month
as they’ve found in the last 20 years.
Finding more of them will help astronomers refine the accuracy of
this method, contribute to an improved 3-dimensional map
of the universe, and better understand how
the universe has expanded and evolved
throughout cosmic history.
[NASA]