Say It With Science

Variable Star Astronomy

Variable stars are stars whose brightness changes because of physical changes within the star. There exist more than 30,000 variable stars in just the Milky Way. Variable star astronomy is a popular part of astronomy because amateur astronomers play a key role. They have submitted thousands of observed data and these data are logged onto a database. American readers can find information on it on the American Association of Variable Star Observers page. 

One of such variable stars are called Cepheids. Cepheids are pulsating variable stars because they undergo  a “repetitive expansion and contraction of their outer layers” [1]. In Cepheids, the star’s period of variation (about 1-70 days) is related to its luminosity; the longer the period, the higher the luminosity. In fact, when graphed, the relationship is shown by a straight line (as can be seen on the title image). Henrietta Swan Leavitt, an American astronomer, first discovered this and understood the significance of this knowledge.  Combined with understanding of the star’s apparent magnitude (a previously written post on this subject can be found here), astronomers can use this information to find a star’s distance from Earth. Cepheids are famously known for their usefulness in finding distances to far-away galaxies and other deep sky objects. Leavitt died early from cancer but was to be nominated for the Nobel Prize in Physics by Professor Mittag-Leffler (Swedish Academy of Sciences). 

Edwin Hubble used Leavitt’s discovery to prove that the Andromeda Galaxy (M31) is not part of the Milky Way, but was able to find the distance to the Andromeda Galaxy (between 2-9 million light years away). At first his calculation was incorrect (900,000 light years) because he observed Type I (classical) Cepheid Stars. Type I Cepheid stars are brighter, newer Population I stars. Hubble later used type II Cepheids (also called W Virginis stars), which are smaller, dimmer, Population II stars, and he was able to make more accurate calculations.

To determine the star’s distance, use the inverse square law of light brightness. 

A similar type of star are RR Lyrae Variable Stars. They are smaller than Cepheids and have a much shorter period (from a few hours to a day). On the other hand, they are far more common. Likewise, they can be used to solve for distances as well. Low mass stars live longer, and thus Cepheid stars are generally younger because they are more massive. 

Both Cepheids and RR Lyrae Variable stars are referred to as standard candles: objects with known luminosity. If you’ve ever wondered how astronomers came to those enormous figures when describing how far away galaxies and stars are from us, you can now better understand why and how. 

Refrigerated Electron Beam Ion Trap (REBIT)

An ion trap is an experimental physics device that captures ions for use in condensed matter experiments. A common design, pictured above, uses extremely strong magnetic fields (on the order of teslas) to accelerate a central electron beam. When a gas is released into the chamber, particles near the beam have their outer electrons ripped off by the magnetic field. The stronger the magnetic field, the more you can ionize your particles.

Refrigerated EBITs can allow the magnetic field to become absurdly strong due to superconducting effects in the magnetic coils. These devices are capable of producing highly charged ions (HCIs). A REBIT can take xenon gas, for example, and give back Xe³⁴⁺. That’s xenon — a noble gas that loves its electrons — with 34 of its 54 usual electrons ripped off! Particles like this are extremely energetic, and surface physics experiments often investigate the interaction of these particles with metallic surfaces. These very angry HCIs can create microscopic craters in a previously clean surface.

Besides surface physics, HCIs are found in several astrophysical systems. This makes REBIT facilities one of the rare places where scientists can perform experimental astrophysics, by generating and experimenting with high charged plasmas in the lab. Highly charged ions can be found in powerful cosmic phenomena like stellar coronae and accretion disks in quasars.

Black Holes: A Mystery of Gravity

Black Holes could be today’s most popular stellar objects. Everyone is fascinated by them. Songs are named after them (see Supermassive Black Hole by Muse). But what is it about these intergalactic mysteries which makes them so cool?

Let’s first consider a star. What makes a star, a star? A star is a plasma that emits light through the universe. The inward pull of gravity in a star is balanced by the outward pressure of nuclear reactions. Further reading on this can be found here. 

But this is not the case for a black hole. A black hole is literally like a “hole”, a hole in the fabric of space-time! A black hole is not limited in its gravitational pull.  It has an almost infinite amount of gravitation, dragging everything, including light inside of it. Thus the name “black”. 

All of this mass and gravitation is condensed down to one single point— a point referred to as a singularity. 

But imagine that you were approaching a black hole. Famous astrophysicist Neil deGrasse Tyson explains what would happen here, on his popular show NOVA scienceNOW (a quick, recommended, fun watch). 

Whilst approaching the black hole, you would pass by the event horizon, a point of no return. Anything within the event horizon, including light, would not be able to escape. You would then undergo a process called spaghettification where the atoms of your body separate and then gather at the singularity.

Death by Black Hole seems like a pretty cool way to make your exit from the world. Hehe. Just kidding. 

Perhaps there might be a black hole in the middle of our own galaxy? And many more? Maybe you can join astrophysicists and go on your own black-hole hunt.

Have you ever looked out on a starry night and wondered what else is out there? Perhaps, who else? And if there were to be someone, something there— would they be looking out for you, too?

Don’t worry, you’re not alone. Others have theorized about it: Frank Drake (an American Radio astronomer who wrote the famous Arecibo message) made an entire equation. Behold, The Drake Equation.

N = R* × f_p × n_e × f_l × f_i × f_c × L

The Drake Equation is an equation for predicting the number of civilizations in the Milky Way Galaxy capable of interstellar communication.

Short descriptions of what the variables of the equation represent can be found here.

The variables represent the average rate of star formation per year in our galaxy, the fraction of those stars which have planets, the average number of planets that can potentially support life per star which has planets, the fraction of those which actually go on to develop life in the future, the fraction of those which go on to develop intelligent life, the fraction of those which can release detectable signals of their existence, and (finally) the length of time for which these civilizations release signals.

That all seems like a mess, but you get the idea.

According to Drake’s parameters:

• 50% of new stars develop planets
• 0.4 planets will be habitable
• 90% of habitable planets develop life
• 10% of new instances of life develop intelligence
• 10% of such life develops interstellar communications
• These civilizations, might, on average, last 10,000 years.

To be fair, we are not sure on the actual figures. Drake’s values gives an answer of 10, meaning that 10 of these theoretical civilizations would be able to communicate.

But the importance of Drake’s equations is not necessarily the numerical value. It lies in all the questions that the equation led him to. Who knows exactly how many stars there are and what not? These figures are yet to be discovered.

So next time you look above, remember to always question. You’re not alone in questioning and you don’t know where these questions can lead you. Like Drake, you might be led to discover companions from different worlds.

The Neutron Star

When a star runs out of fuel it eventually goes through a gravitational collapse. There are several possible outcomes, and three come about by simply taking into account only the mass of the star that has just collapsed. If the star is less than 1.5 solar masses, then a white dwarf is formed. Yet if its mass is larger than 5 solar masses, it will create a black hole. What about if the star’s mass was in between those two points? Well, that’s when a neutron star is formed. Due to the inward collapse of such a star, electrons combine with protons to form neutrons – thus giving the resultant celestial body the name “neutron star”.

Neutron stars characteristically have extremely high densities. Anything falling into a neutron star is super-accelerated by gravity (which is 100 billion times stronger than what we experience on earth). “If you dropped a marshmallow onto a neutron star, it would have the energy of an atomic bomb,” says Chip Meegan from the Marshal Space Flight Center of NASA. Neutron stars also have insanely strong magnetic fields, approximately 2 x 10¹¹ times those of Earth. These stars are usually very hot. The degeneracy pressure due to the Pauli exclusion principle (no two neutrons or any other fermions can occupy the same place and quantum state simultaneously) ensures the neutron star’s stability and prevents it from collapse. (The only situation in which a neutron star would collapse into a black hole is if it is gradually absorbing matter from an accompanying binary star.)

Pulsars!

Very simply, pulsars are rotating neutron stars. Because they conserve the angular momentum of the stars from which they were formed, pulsars can spin at rates over 700 times revolutions per second. They stream jets of highly energetic particles (which have speeds close to that of light) out from their magnetic poles, producing extremely powerful beams of radiation. This combined with their rotational movement allows them to appear as if they are pulsing when they are observed. The rotational and magnetic axes of these stars are misaligned, which causes the beam of light from the jets to “sweep” around as the pulsar rotates, giving rise to the lighthouse effect.

The H-R Diagram

• named after Danish and American astronomers Ejnar Hertzsprung and Henry Russel
• a graphical representation of the relationship between the luminosity, surface temperature, and radius of stars
• it is known to be the most important diagram in astronomy as it shows the state of a star throughout its life

Absolute Magnitude of a Star

Out of all the stars that we observe at night, it is easy to identify a certain few. Is it because they look brighter to us?

As observers on earth, we only notice the apparent magnitude (m) of the star. 

Astronomers often need to know how much light a star truly radiates into space. This is referred to as the star’s luminosity.The stars we see in the sky are oftentimes different distances away from us. 

Let’s imagine 2 stars which look equally bright to us. The further star has a greater luminosity, a greater absolute magnitude (M). The absolute magnitude of a star is the apparent magnitude that it would have if it were observed at a distance of 10 parsecs.

To fairly compare stars at different distances, scientists use the absolute magnitude formula:

M = m-5log(d/10)

Where:

M= absolute magnitude

m= apparent magnitude 

d= distance in parsecs*

*Note: A parsec is equivalent to 3.26 light years

Image is Star Cluster NGC 1850. [Image source]