Variable Stars

So, what are variable stars? In simple words, a star whose brightness changes is called variable star.
This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are classified as either:

• Intrinsic variables, whose luminosity actually changes; for example, because the star periodically swells and shrinks.
• Extrinsic variables, whose apparent changes in brightness are due to changes in the amount of their light that can reach Earth; for example, because the star has an orbiting companion that sometimes eclipses it.

Cepheid variable stars in Trifid Nebula

Variable stars are generally analysed using photometry, spectro-photometry and spectroscopy. Measurements of their changes in brightness can be plotted to produce light curves. For regular variables, the period of variation and its amplitude can be very well established; for many variable stars, though, these quantities may vary slowly over time, or even from one period to the next. Peak brightnesses in the light curve are known as maxima, while troughs are known as minima.

The light curves give information about the periodicity of the brightness and the shape(symmetrical, or not, angular or smoothly varying). The spectrum tells us about the star type, temperature and luminosity class(dwarf, giant, supergiant), & if it is single or binary star. We can also know if there's any change in spectrum, effect of magnetic fields, and the reason for abnormal absorption or emission, if any. Conclusions are drawn by combining the data collected from the light curves and the spectrum.

These variable stars can be classified into: 1. Intrinsic variable stars, 2. Extrinsic variable stars.
For examples, link: https://en.wikipedia.org/wiki/Variable_star#Classification

Eta Carinae, a variable star, in Carina Nebula

Pulsars

What are pulsars? Pulsars are flickering stars. No this is not correct. Pulsars are not stars, indeed they are dead stars and they do not actually flicker or pulse.

Pulsars are neutron stars which radiates two steady narrow beam of electromagnetic radiations (light, radiowave, etc.) in opposite directions. Neutron stars form when a star more massive than the sun runs out of fuel in its core and collapses in on itself(Neutron stars will be explained in future posts). 

Neutron stars are typically about 20 to 24 km in diameter, but they can contain up to twice the mass of the sun, which is about 1.392 million km in diameter. A sugar-cube-size bit of material from a neutron star would weigh about 1 billion tons — "about the same as Mount Everest," according to NASA. The only object with a higher density than a neutron star is a black hole. Pulsars are highly magnetic. Pulsars have magnetic fields that range from 100 million times to 1 quadrillion (a million billion) times stronger than Earth's.

Why do pulsars seem to flicker? The reason is quite simple. It's similar to why a lighthouse seem to flicker. The axis of rotation of pulsar and the line in which light beams come out are at an angle. So as the pulsar spins, these light rays starts falling on earth then vanishes and then again appear, because of this phenomenon pulsars seem to flicker.

Why do pulsars spin? Because pulsars are formed from stars and we know that stars spin on their axis. Why pulsars spins faster than stars? Pulsars are neutron stars which form when stars collapses. When a star collapses its radius decreases so the whole mass gets concentrated near the axis of rotation and now to conserve the angular momentum, the neutron star's angular velocity increases. That's why pulsars spin so fast about hundreds of time pr second.

Why pulsars radiate? There is no correct explanation for it yet. But there is a theory why it radiates light. The theory goes like this- Pulsars are highly magnetic and a spinning magnetic field generates an electric field, which, in turn, can cause charged particles to move. At the poles where magnetic fields dominates the surface, charged particles like electrons and protons, or charged atoms, are accelerated to extremely high speeds by the very strong electric field. Any time charged particles are accelerated, they radiate light. Why pulsars radiate radiowaves and gamma-rays? The answer to this question is still unknown.

Shape of the Universe

So familiar with the term universe, but ever wondered about the shape of it?

Yeah, you read right. and the universe has a shape. Sounds a little queer, right. To be true, it is strange. It cannot be explained directly by the shapes we know. It is related to general relativity, which describes how spacetime is curved and bent by mass and energy.

The shape of the universe, is the geometry of the universe. The geometry can be broadly divided as i)local geometry, and ii)global geometry.

We make it clear, that universe, when talked of, can mean the observable universe, and the global universe. The local universe, is particularly the global universe. We don't know if the observable universe, is the global universe.

The observable universe,that is related to the curvature, as said earlier, can be explained by, the FLRW Model, which states, in layman terms that, if dark matter(will be explained in future posts) is ignored, the curvature can be determined from the average density of the universe, assuming matter is evenly distributed.
for more information, I'll leave the link for FLRW here-
 https://en.wikipedia.org/wiki/Friedmann–Lemaître–Robertson–Walker_metric

Well things aren't that straight-forward with global universe, there are questions, intriguing ones.
Is the universe finite, or infinite?
Is there a boundary, if yes, what is beyond that?

The finiteness of the universe is unanswered till now. The boundary, yeah, is unresolved. The universe may have an edge, maybe not.

This is an interesting topic, well, all topics about our universe, are. Many areas are yet to be covered. These complexities of universe inspire us to learn more about it and fascinated us to start this blog sharing our views on universe.
For a mathematical approach to the shape, you can check the book - https://en.wikipedia.org/wiki/Our_Mathematical_Universe

Black Hole

What are black holes? How are they formed? Let's see.

Black holes are specific regions in spacetime with extremely intense gravitational field that doesn't let any particle and even electromagnetic radiations like light to escape. And because light gets pulled into the black hole they appear black.

Black holes are formed when big stars collapses. As the star ages, it finishes up its fuel leaving extremely heavy elements in its core. As the core gets heavier and heavier, it eventually becomes so heavy that it collapses under its own gravitational pull. While this happens it releases lot of energy and mass into space in the form of an explosion we call Supernova explosion. As time passes the core sucks in more mass and then its gravitational pull becomes so strong that it even sucks in light and spacetime. We call this core and surrounding space as black hole.

A black hole has two parts, one which we can see-the region surrounding the black hole consisting of hot gases and dust which came from the stars and planets it sucked in. The other part is the one which we can't see- the inner core where gravity doesn't let even light to escape. These two parts are separated by a fictitious wall called Event Horizon.

Let's think we throw some object with a clock and camera fitted in it towards a black hole and somehow it communicates with us all the time. As the object approaches the black hole the gravitational pull on it keeps on increasing. Moreover as it goes near the event horizon the difference in the gravitational pull between the front and back faces of the object increases which gives more acceleration to the front face than the rear one which cause it to elongate. Gravitational pull of black hole is so strong that the object elongates to form a extremely long and thin string type thing. This phenomenon is known as the 'spaghetti effect'.

We assume that still the object communicates with us. One more thing that happens as it nears the event horizon is that the black hole warps spacetime around itself which causes huge time dilation. Why this is caused? Because the light emitted by the object find it tougher and tougher to cross the gravitational pull. The clock inside the object starts ticking slowly. And as soon it reaches the Event horizon time almost stops. We will see that the object suddenly stops there and stays there forever. We know that electromagnetic radiations can't escape black holes but we assume that somehow the object is communicating with us in realtime. As the object crosses the event horizon suddenly we can see all the light that is being pulled inside. This is a big peculiarity that the light inside black hole maybe a few billion years old. We can see our universe evolving from the past in front of us. Eventually we will fall in the center called the singularity which occupies only an infinitely small space. At singularity the density tends infinity and with so much of gravitational pull at this point many laws of physics breaks down. Our human imagination stops working at understanding the point of singularity.

Are we alone?

Are we alone? Hm... I think the answer is most probably no. First lets see why we are here?

We are here in our home planet Earth which has everything in perfect amount to support life. Earth has the perfect amount of water to support life and have seasons. Earth has perfect amount of oxygen needed to support life, if there was more oxygen we couldn't control fire, if it was less we would die. Earth has perfect amount of carbon dioxide to keep the Earth warm or else I would be typing this post from a house full of ice because if Earth gets cold enough Ice Age will come. And many more such elements present in perfect amount supports life. How Earth has these elements in such precise amount? By luck that's all what I can say. Can there be another planet-star system like our solar system which contains a planet like Earth? Maybe.

Our Universe is large, extremely large. We usually say that universe is infinitely large. When Hubble focused in very tiny potion of sky(comparable to size of finger-nail), it took a famous photo of lots of galaxies in it.

Hubble Deep-Field

This photo contains almost 10,000 galaxies in it. Our own milky-way galaxy homes almost 300 billion stars. So you can think of the total no. of stars in this universe. In this infinitely large universe there can be star contain a planet at the perfect distance with all the elements in same amount as that of Earth which can support, or supports life.
If we are not alone, then why we don't know about those intelligent life species? The reason lies in the extremely large galactic distances. The farthest radio signal sent by humans have reached about 200 light years of distance, Wow!!! No, nothing so special. In our universe 200 light years is nothing where the diameter of our Milky-way galaxy is about 100,000 light years which is itself one of the billions of galaxies in our universe. There is a picture depicting how small is 200 light years.

No it is not the black box, it is the blue dot in the zoomed portion. Click on the image and see in full resolution. Let's think that there is an intelligent life species somewhere in the opposite end our galaxy. The signal we will send today will reach there after 100,000 years. In 100k years the intelligent life species may be no more there(due to catastrophe they die), let's think they will be there and listening for some signal and they receive it. In reply they will send a signal that will reach us after more 100,000 years. Because of this huge galactic distances till now we couldn't find them. But someday we will surely find other intelligent life species in our universe.

Birth of Our Universe

So lets talk about the birth of our universe. How was it formed? By Big-bang. Now what is this Big-bang? Why it happened? How it happened? Let's find out.

BIG BANG

According to our knowledge and accuracy, our universe was formed some 13.7 billion years ago in a massive explosion which burst out everything, every single element present in our universe that we know as Big-bang.

So what caused this big-bang? Currently most scientists agree on this hypothesis that big-bang was caused by an infinitely small particle of infinite density that was so much unstable that it released it's energy in this massive explosion. This explosion released so much energy that elements fused and disintegrated to form new elements creating everything present around us.

Now how this tiny energetic particle formed? Nobody knows. This is the point where human imagination stops working. This is why scientists say that time started ticking at the instant of big-bang. So, how we know about that extremely tiny energetic particle? Again this concept is totally hypothetical. From decades scientists are locating some galaxies far distant from us and what they found is that our universe is expanding. All the galaxies are going far from each other as if they started from a same point. So if we go back in time it may be possible that all these galaxies fuse together and create a body of huge mass whose mass goes on increasing and then because of its huge gravitational force the mass collapses into an infinitely small and infinitely dense particle which later caused the big-bang.

Introduction

Universe?? Or Multiverse??  What do you think which one is correct? We will talk on this later. In this blogspot, I, Abhishek Adhikari, a teen from India who loves the complexity in understanding what we are, where we are and how are we here, will share my thoughts on these topics. From childhood I love understanding our universe. In this blogspot, every week I will upload atleast two posts sharing our human knowledge in understanding our universe.