China's Sky Eye, moving towards 1,000 pulsars!

In recent days, a piece of news about China's world-famous "big pot" has appeared on major news platforms - the "China Sky Eye" FAST has discovered 883 pulsars and plans to refresh this number to 1,000 within the year.

Image from CCTV News Channel

So, what exactly are pulsars? Why do we go to so much trouble to find them? And what are they good for once we find them? Today, let's take a closer look and share some gossip about FAST.

Does a pulsar mean a star that flashes like crazy?

Let's talk about "pulsars" first. From the perspective of the Earth, pulsars are celestial bodies that periodically flash electromagnetic pulses, with extremely short pulse intervals ranging from a few milliseconds to hundreds of seconds. However, pulsars are not really flashing , the so-called pulses are just an illusion caused by the pulsar's crazy rotation.
So how do pulsars come about? They are actually the result of the "inner pull" of stars.

Every "normal" star that we can see with our naked eyes has two forces working against each other : gravity drives matter in toward the core, while energy released by nuclear fusion pushes matter outward.

The fuel for nuclear fusion will eventually run out, so gravity will eventually win the duel . When a massive star (for example, more than 8 times the mass of the sun) finally uses up all its fuel, it collapses inward, implodes violently, and then spreads outward in a brilliant display of fireworks. This process is called a supernova explosion.

In the first year of the Northern Song Dynasty (1054), a supernova exploded near the "Tianguan" constellation in Taurus. It was visible for 23 days during the day and 22 months at night. This supernova explosion was recorded by Chinese astronomers and was called " Tianguan Guest Star " in history.

When the dust and smoke clear, a very dense celestial body may be left at the original position of the star - a neutron star . Inside it, the atomic structure no longer exists, and electrons are squeezed into the nucleus and combined with protons to form neutrons. The mass of a neutron star exceeds 1.4 suns, but its diameter is only a dozen kilometers. In other words, every cubic centimeter of neutron star matter is equivalent to the total mass of the world's human beings!

Neutron stars also inherit the rotational angular momentum of the residual mass of stars. Under the same angular momentum, the rotation speed is inversely proportional to the square of the radius. We often see ice dancers folding their arms or raising them above their heads while spinning, and they will suddenly spin very fast. Similarly, when a star collapses into a neutron star, the rotation speed will soar by a hundred million times.

Radio pulses from a pulsar sweep past Earth. Photo by Michael Kramer

Neutron stars have strong magnetic fields that drive the charged particles around them, emitting strong beams of radio radiation that gush out from their two magnetic poles. If the radiation beam that happens to sweep across the Earth as the neutron star rotates, we can detect periodic radio pulses, just like the special effects lights in some discotheques that always spin in circles. Although the lights are always on, they are sometimes bright and sometimes dim when viewed from one direction. Well, with this analogy, pulsars can be said to be the remains of stars dancing on their own graves...

The Tianguan guest star mentioned above left behind a pulsar with a period of 33 milliseconds ( 30 rotations per second ), and the cooling fireworks it emitted are the famous Crab Nebula .

The Crab Nebula. Image source: NASA

Among the more than 3,000 pulsars discovered worldwide, the vast majority are neutron stars, but there are also two that are white dwarfs (the remains of low-mass stars that still retain their atomic structure): AR Scorpii and AE Aquarii.

FAST does not mean "fast"

Most pulsars do not emit significant radiation in the visible light band, but appear brighter in the radio band. Fortunately, on Earth, the atmosphere is quite favorable to the radio band and has extremely high transparency, so radio telescopes are particularly suitable for observing pulsars on the ground.

The shielding effect of the Earth's atmosphere on electromagnetic waves of various wavelengths. Image source: NASA

Next, let’s talk about our FAST.

FAST is named after the Five-hundred-meter Aperture Spherical radio Telescope. This giant single-dish radio telescope is located in Dawodang, Pingtang County, Guizhou Province. It was built in accordance with the natural depression of karst landform. Construction began in 2011 and was completed in 2016. It is currently the world's largest radio telescope with a full-aperture reflective surface (Russia's RATAN-600 has a diameter of 576 meters, but only a thin circle of reflective rings).

Aerial view of FAST. Image source: FAST official website

——By the way, you may think that the abbreviation FAST sounds cool, but the full name seems too straightforward. There is no way, "the abbreviation is unclear and powerful, and the full name is really uncreative" is a tradition in the astronomical community , such as TMT is "Thirty Meter Telescope", VLT is "Very Large Telescope", ELT is "Extremely Large Telescope", EELT is "European Extremely Large Telescope". Does the Webb Space Telescope sound normal? But its original name was actually "Next Generation Space Telescope" (relative to Hubble)...

Why are radio telescopes so large? This is because under the same resolution requirement, the longer the wavelength to be observed, the larger the aperture of the "dish" must be, otherwise it will not be clear . The Webb telescope, which works in the infrared band, has a larger aperture than the Hubble telescope, which focuses on visible light (6.5 meters vs. 2.4 meters), and the band that radio telescopes need to observe is 5 or 6 orders of magnitude higher than these two. It is really necessary to make it bigger. The aperture is justice. It is absolutely correct to use it here.

Careful readers may have two questions:

① A spherical surface cannot actually focus distant starlight onto a single focus; a parabola is required. So why is FAST made into a spherical telescope?

② If a big pot is placed on the ground like this, wouldn't it only be staring at the zenith? Even if the earth rotates, it can only scan the circle where the zenith is?

In fact, this is a common misunderstanding, and it is also a negative effect of using simple analogies in popular science. Because of their shapes, we like to call various radio telescopes "pots". However, this will also mislead our thinking, and we may easily think that FAST is like the big iron pot we use for cooking at home, which is hard and fixed, and its shape will not change. But in fact, FAST is very flexible.

FAST is made up of 4,450 reflective panels that are driven by motors and can change their posture. When the reflective panels in an area are adjusted regularly under unified command, they can create "ripples" in the "pot" and change the shape of the mirror.

According to calculations by Nan Rendong, the "Father of FAST", and his team, a deviation of 0.47 meters from the spherical surface can transform a 300-meter-diameter spherical surface into a parabola and focus the radio signal on one point. Therefore, at any time, FAST has only a circular working area with a diameter of 300 meters . Through the concerted adjustment of the reflector, this working area can "drift" freely in the "dish", so the range of observable sky areas is quite wide.

If the 300-meter aperture is maintained, it can cover the sky from 52.2°N (the working area is close to the south edge of the dish) to 0.6°S (the working area is close to the north edge of the dish). If you are willing to sacrifice a little effective aperture, you can cover the sky from 65.8°N to 14.2°S.

FAST optical path, the yellow dotted line is the parabola working area. Image source: Nan Rendong "FAST Project Introduction"

What are the practical applications of observing pulsars?

FAST has discovered so many pulsars, so what are the practical applications of observing pulsars? It has quite a lot of uses.

When the signal from a pulsar travels through the interstellar space, it will be blocked by the ionized gas along the way, causing delays. The longer the distance, the more ionized gas there is, and the greater the delay. If we know how far the pulsar is from us, and then precisely measure the degree of delay, we can infer the distribution of the interstellar medium along the signal's path.

The magnetic field also affects pulsar signals. When an electromagnetic signal passes through a magnetic field, its polarization properties will be changed. The stronger the magnetic field, the greater the change. Measuring the polarization of the signal can infer the distribution of the magnetic field along the signal.

When a supermassive object disturbs space-time, gravitational waves are generated, changing the time it takes for the pulsar signal to reach us. So by accurately measuring the fluctuations in the pulsar period, gravitational waves can be detected . If we can discover a pulsar-black hole binary system and observe how a celestial body with stable output and a celestial body that distorts space-time stir up the universe, we can better test the predictions of general relativity and greatly promote basic physics research .

The rotation period of pulsars is very stable, and some of them can be comparable to atomic clocks in long-term performance. Moreover, they are "always powered on" and much more durable than atomic clocks. Combining pulsars and atomic clocks can establish a long-term stable and accurate time system, which can even be used for interstellar navigation.

The Voyager "Voice of Earth" Golden Record uses 14 pulsars to indicate the position of the solar system in the lower left corner. Image source: NASA

To sum up, FAST and the pulsars it discovers will help us better understand the universe, and these discoveries may one day help humans navigate in the sea of ​​stars.

Planning and production

Author: Qu Jiong Popular Science Writer

Review丨Liu Qian, Researcher at Beijing Planetarium

Planning丨Ding Zong

Editor: Ding Zong