New Scientist - USA (2019-12-21)

56 | New Scientist | 21/28 December 2019

Stuart Clark is a freelance

writer based in Hertfordshire,

UK. He is an ageing star who

occasionally explodes

NA

SA

“ Stars can only

go supernova

once – but one

seems to explode

again and again”

Cassiopeia A

is among the

best-studied

supernovae,

but far from

the strangest

runs out of fuel for its nuclear reactions. By

definition, it can only happen once to any

given star. Imagine the shock, then, when Iair

Arcavi, then at Las Cumbres Observatory in

California, found a star that seemed to explode

over and over again.

Supernova iPTF14hls was discovered in 2014.

Over the course of almost two years, Arcavi and

his colleagues watched iPTF14hls flare up no

less than five times. Archive images from the

Palomar Observatory in California show that

the star could well have been exploding as

early as 1954, but that it wasn’t in 1993, when

another image of its host galaxy was taken.

Perhaps the strangest thing was that

analysis of the light coming from the star

always suggested it was a completely normal,

freshly exploded supernova, even hundreds

of days into the explosion. Then, just when

the researchers were getting used to its

persistence, the supernova disappeared. “It’s

the weirdest supernova we’ve ever seen,” says

Arcavi, now at Tel Aviv University in Israel.

There is currently no conclusive explanation.

The best stab so far is that the star is so

enormous that its interior is hot enough to

form antimatter, triggering an explosion.

Arcavi’s team suggests that this process has

happened several times in the past, leading not

to the complete destruction of the star, but to a

series of eruptions in which shells of gas were

ejected into space. The most recent explosion

then blasted out a final shock wave of material

that caught up and collided with the other

ejected shells one after another, creating the

fresh-looking flares observed from Earth.

The trouble is, the first shell released should

carry away all the hydrogen gas. Yet in the case

of iPTF14hls, every time the light peak rose,

the team saw the chemical signature of this

element. To figure it out, Arcavi wants to find

similar supernovae. But perhaps he should just

wait, because the 2014 explosion may not be

the last. “Maybe it’s still there, and it will

surprise us again in 10, 50 or 60 years’

time,” he says.

The oldest star in the cosmos?

There is a star that seems to be older than the

universe, unless the universe is older than

we thought. HD 140283, around 200 light

years from Earth, is made almost entirely of

hydrogen and helium. This suggests it is one

of the very earliest stars to form because stars

reveal their approximate ages by the amount

of “metals” – the astronomical term for any

element heavier than helium – they contain.

Estimates of HD 140283’s age come in at

around 14.46 billion years, whereas the

universe is thought to be around 13.8 billion

years old. And there must have been at least

one previous generation of stars to have

produced the metals in this star. The upshot

is that either the cosmology used to calculate

the universe’s age or the stellar astrophysics

used to calculate the star’s age is wrong.

Howard Bond at the Space Telescope Science

Institute in Maryland, who did the estimations,

thinks they could be out by 800 million years

either way. That could bring the star’s age down

to fit within the age of the universe, but it

would leave precious little time for the first

generation of stars to live and die. All of which

goes to show how little we know about the

early days of the cosmos.

The one so big it would

engulf Jupiter

When it comes to deciding on the biggest star

in the galaxy, there are some gargantuan

contenders. In 2013, astronomers measured

three red supergiant stars. They found that all

three were 1000 times larger than the sun, but

the winner was UY Scuti.

This one is actually a hypergiant, because

estimates suggest it is 1708 times larger than

the sun. If it were to replace our star, it would

reach most of the way to Saturn’s orbit. And

talk about bright: UY Scuti is 300,000 times

more luminous than the sun.

Its status as the biggest of the big is disputed,

however. Another group of astronomers

thinks the star is much closer to Earth than

is generally thought. If so, UY Scuti must be

smaller, perhaps just 825 times the sun’s size.

Even at its largest estimate, UY Scuti could

still be dwarfed by the star WOH G64, which

resides in a nearby galaxy, the Large Magellanic

Cloud. Estimates place it somewhere between

1540 and a whopping 2575 times the size of the

sun, although its greater distance from us

makes it even harder to measure accurately.

The star that eats its twin

Algol is one of the only variable stars in the

night sky noticeable to the naked eye. Every

2.867 days, it fades for 10 hours, then returns to

normal – a wink that led the ancient Greeks to

draw the constellation of Perseus around it and

label Algol the eye of the snake-haired Medusa.

We now understand that Algol is part of a

system in which two stars regularly eclipse

each other, but the 2.867-day orbit still presents

a puzzle. To travel that fast around each other,

the stars should be so close that one pulls gas

from the other. This would slowly push the pair

away from one another, lengthening the orbit,

but astronomers haven’t seen any sign of this.

It turns out the ancients may provide the

answer. Sebastian Porceddu at the University

of Helsinki in Finland believes that the ancient

Egyptians based some of their calendar’s

unlucky days on Algol’s eclipses. The

periodicity he found was about 20 minutes

shorter than the Algol system today – an

amount that could easily be explained by

one star pulling gas from the other. ❚