Gravitational waves

Description

Context

Gravitational waves revealed the first known mergers of a black hole and neutron star.
Until now, all identified sources of gravitational waves were two of a kind: either two black holes or two neutron stars, spiraling around one another before colliding and coalescing.

Gravitational Waves

A gravitational wave is an invisible (yet incredibly fast) ripple in space.
Gravitational waves travel at the speed of light (186,000 miles per second).
These waves squeeze and stretch anything in their path as they pass by.

What causes gravitational waves?

The most powerful gravitational waves are created when objects move at very high speeds. Some examples of events that could cause a gravitational wave are:
when a star explodes asymmetrically (called a supernova)
when two big stars orbit each other
when two black holes orbit each other and merge.
But these types of objects that create gravitational waves are far away.
And sometimes, these events only cause small, weak gravitational waves.
The waves are then very weak by the time they reach Earth. This makes gravitational waves hard to detect.

How do we know that gravitational waves exist?

In 2015, scientists detected gravitational waves for the very first time.
They used a very sensitive instrument called LIGO (Laser Interferometer Gravitational-Wave Observatory).
These first gravitational waves happened when two black holes crashed into one another.
The collision happened 1.3 billion years ago. But, the ripples didn’t make it to Earth until 2015!

How are gravitational waves detected?

When a gravitational wave passes by Earth, it squeezes and stretches space.
LIGO can detect this squeezing and stretching.
Each LIGO observatory has two “arms” that are each more than 2 miles (4 kilometers) long.
A passing gravitational wave causes the length of the arms to change slightly.
The observatory uses lasers, mirrors, and extremely sensitive instruments to detect these tiny changes.

Neutron Stars

· Neutron stars are formed when a massive star runs out of fuel and collapses.

· The very central region of the star – the core – collapses, crushing together every proton and electron into a neutron.

· If the core of the collapsing star is between about 1 and 3 solar masses, these newly-created neutrons can stop the collapse, leaving behind a neutron star. Once formed, they no longer actively generate heat, and cool over time.

· Neutron stars are the smallest and densest currently known class of stellar objects.

· Since neutron stars began their existence as stars, they are found scattered throughout the galaxy in the same places where we find stars. And like stars, they can be found by themselves or in binary systems with a companion.

· Many neutron stars are undetectable because they simply do not emit enough radiation. A handful of neutron stars have been found sitting at the centers of supernova remnants quietly emitting X-rays.

· In binary systems, some neutron stars can be found accreting materials from their companions, emitting electromagnetic radiation powered by the gravitational energy of the accreting material.

· The two general classes of non-quiet neutron stars are – pulsars and magnetars.

https://www.sciencenews.org/article/gravitational-waves-ligo-first-black-hole-neutron-star-merger