​Time travel happens every day. You just don't realise it because it happens on such a minuscule scale.

No, I've not finally lost the plot, time travel into the future is possible. Let's look at an example. Ironman is walking along a (cross-franchise) rocket. He whizzes past a Stormtrooper. 

If Ironman is walking a 2 mph, he thinks his speed is 2 mph. But the Stormtrooper sees his speed as 2mph PLUS the speed of the rocket.

This concept doesn't just work for speed, it also works for time. Basically, the amount of time that passes between two events depends on your point of reference.

When you move really fast and approach the speed of light, the slower time passes for you. It's an effect called time dilation.

Essentially, the faster you move through space, the slower you move through time.

So, if you went on a spaceship travelling at close to the speed of light, then came back to Earth - you'd be returning to a future Earth where time has passed faster on the planet than in your spaceship.

Gravity also messes with the passage of time. The greater the gravity, the greater your speed and (therefore) the slower you'll move in time.

If, for example, you were in a spaceship and decided to go on a day trip to a black hole (which massively warps the fabric of space-time) then time would pass quicker, again, for you compared to someone further away from the black hole.

Theoretically, yes. But you'd need something called a wormhole, which essentially acts as a bridge through time and space to connect two distant points with a shortcut.

It's still a topic of great debate in the scientific community, but one of the major constraints is that you need to create a wormhole to travel in time. So, you can only travel forwards in time - because even if we created a wormhole capable of carrying humans now, no one has created one in the past for us to travel back to.

​Oh, and you'd need to be travelling a near to the speed of light for all of this to work.

​Confused? Don't worry - I'll look more into wormholes next week!

If you're not convinced about time dilation, then this experiment finally proved the phenomenon a few years ago. And did you know that the clocks on the International Space Station (hurtling around the Earth at more than 17,000 mph) also tick a little bit more slowly than the clocks on Earth?

The premise of time travel is also deeply rooted in two of Einstein's theories that I've covered in previous Sunday Science posts: General Relativity and Special Relativity.

If you want to read more about wormholes and time travel into the future, this is a great article.

Finally, Brian Cox explained time travel as part of a special Science of Doctor Who lecture a few years ago. You can check out the video clips here.

When a suitably massive star comes to the end of its life, it collapses in on itself in a massive supernova explosion. If that original star was between 10 and 29 solar masses it collapses to the point where the protons and electrons in its core are squashed together leaving behind a neutron star.

Neutron stars are incredibly small and incredibly dense. They cram roughly 1.3 to 2.5 solar masses into a sphere of roughly a few miles in diameter.

If we could extract just one sugar-cube sized piece of a neutron star, it would weigh more than one billion tons. That's equivalent to the weight of Mount Everest.

If the neutron star weighs more than three solar masses, it will continue to collapse in on itself to form a black hole.

Neutron stars also have very large magnetic fields (approximately one trillion times stronger than the Earth's magnetic field) and are the strongest magnets in the universe. They have a surface temperature of one million Kelvin (the Sun's surface temperature is 5,778 Kelvin).

Magnetars are another type of neutron star. The magnetic field of a neutron star is one trillion times that on Earth, but the magnetic field of a magnetar is another 1,000 times stronger.

In all neutron stars, the star's crust is linked to its magnetic field so that any change in one affects the other.

So, a small movement in the crust will cause ripples in the magnetic field. For a magnetar, such movement cause a huge burst of electromagnetic radiation. In December 2004, a massive magnetar blast blinded satellites and partially ionised the Earth's atmosphere. The cosmic blast only lasted one-tenth of a second but it released more energy than the Sun has emitted in 100,000 years.

* No Ironmen were harmed in the making of this Sunday Science.

When two neutron stars smashed into each other in August, we weren't quite sure what was left over. But it's looking increasingly likely that it could be the biggest neutron star we've ever detected. The unprecedented celestial collision produced a gamma-ray burst and astronomers at LIGO detected the resulting gravitational waves.

If you'd like to find out more about why pulsars spin so fast and so regularly, you can read more here.

The story of the discovery of pulsars is also one of my favourite astronomy anecdotes as it features a great female scientist, Jocelyn Burnell (nee Bell). I've copied and pasted this fantastic article from the American Society of Physics covering the discovery:

"In 1967, when Jocelyn Bell, then a graduate student in astronomy, noticed a strange “bit of scruff” in the data coming from her radio telescope, she and her advisor Anthony Hewish initially thought they might have detected a signal from an extraterrestrial civilization. It turned out not be aliens, but it was still quite exciting: they had discovered the first pulsar. They announced their discovery in February 1968.

Bell, who was born in Ireland in 1943, was inspired by her high school physics teacher to study science, and went to Cambridge to pursue her Ph.D. in astronomy. Bell’s project, with advisor Anthony Hewish, involved using a new technique, interplanetary scintillation, to observe quasars. Because quasars scintillate more than other objects, Hewish thought the technique would be a good way to study them, and he designed a radio telescope to do so.

Working at the Mullard Radio Astronomy Observatory, near Cambridge, starting in 1965 Bell spent about two years building the new telescope, with the help of several other students. Together they hammered over 1000 posts, strung over 2000 dipole antennas between them, and connected it all up with 120 miles of wire and cable. The finished telescope covered an area of about four and a half acres.

They started operating the telescope in July 1967, while construction was still going on. Bell had responsibility for operating the telescope and analyzing the data — nearly 100 feet of paper every day–by hand. She soon learned to recognize scintillating sources and interference.

Within a few weeks Bell noticed something odd in the data, what she called a bit of “scruff.” The signal didn’t look quite like a scintillating source or like manmade interference. She soon realized it was a regular signal, consistently coming from the same patch of sky.

No known natural sources would produce such a signal. Bell and Hewish began to rule out various sources of human interference, including other radio astronomers, radar reflected off the moon, television signals, orbiting satellites, and even possible effects from a large corrugated metal building near the telescope. None of those could explain the strange signal.

The signal, a series of sharp pulses that came every 1.3 seconds, seemed too fast to be coming from anything like a star. Bell and Hewish jokingly called the new source LGM-1, for “Little Green Men.” (It was later renamed.)

But soon they managed to rule out extraterrestrial life as the source of the signal, when Bell noticed another similar signal, this time a series of pulses arriving 1.2 seconds apart, coming from an entirely different area of the sky. It seemed quite unlikely that two separate groups of aliens were trying to communicate with them at the same time, from completely different locations. Over Christmas 1967, Bell noticed two more such bits of scruff, bringing the total to four.

By the end of January, Bell and Hewish submitted a paper to Nature describing the first pulsar. In February, a few days before the paper was published, Hewish gave a seminar in Cambridge to announce the discovery, though they still had not determined the nature of the source.

The announcement caused quite a stir. The press jumped on the story–the possible finding of extraterrestrial life was too hard to resist. They became even more excited when they learned that a woman was involved in the discovery. Bell later recalled the media attention in a speech about the discovery: “I had my photograph taken standing on a bank, sitting on a bank, standing on a bank examining bogus records, sitting on a bank examining bogus records. Meanwhile the journalists were asking relevant questions like was I taller than or not quite as tall as Princess Margaret, and how many boyfriends did I have at a time?”

Other astronomers were also energized by the finding, and joined in a race to discover more pulsars and to figure out what these strange sources were. By the end of 1968, dozens of pulsars had been detected. Soon Thomas Gold showed that pulsars are actually rapidly rotating neutron stars. Neutron stars were predicted in 1933, but not detected until the discovery of pulsars. These extremely dense stars, which form from the collapsed remnants of massive stars after a supernova, have strong magnetic fields that are not aligned with the star’s rotation axis. The strong field and rapid rotation produces a beam of radiation that sweeps around as the star spins. On Earth, we see this as a series of pulses as the neutron star rotates, like a beam of light from a lighthouse.

After discovering the first pulsars, Jocelyn Bell finished her analysis of radio sources, completed her PhD, got married and changed her name to Burnell. She left radio astronomy for gamma ray astronomy and then x-ray astronomy, though her career was hindered by her husband’s frequent moves and her decision to work part time while raising her son. Anthony Hewish won the Nobel Prize in 1974 for the discovery of the first pulsars. Over 1000 pulsars are now known.**

As for little green men, they haven’t been found yet, but projects such as the Search for Extra Terrestrial Intelligence (SETI) are still looking for them."
​
** Now, more than 2,000 pulsars have been discovered, but this figure was accurate in 2006 when the article was originally published.

If a star has enough mass, it will explode in a monumental supernova explosion at the end of its life.

When a supernova occurs, it burns brighter than an entire galaxy and radiates more energy than our Sun will in its entire lifetime, all within just over a minute. According to NASA, supernovae are “the largest explosion that takes place in space.”

​For a supernova to happen, the star must be between eight and 15 solar masses. One supernova also occurs roughly every second in our universe, but you can't see them all. In fact, a supernova occurs in our galaxy once roughly every 50 years.

Most of the universe's chemical elements are also made in a supernova thanks to the phenomenal heat and pressure in these explosions.

There are two ways a star can go supernova.

A star can pull matter from a nearby neighbour until it has sufficient mass that a runaway nuclear reaction starts and ends in a supernova explosion.

For a Type I supernovae, a White Dwarf usually pulls matter from its companion star in a binary system and builds up enough mass to go supernova.

Type 1 supernovae are thought to blaze with equal brightness at their peaks, so they are used by astronomers as "standard candles" to measure cosmic distances.

A large star runs out of nuclear fuel and collapses under its own gravity. But, instead of ballooning to a Red Giant and contracting to a White Dwarf, it explodes in a supernova. 

Before it reaches the supernova stage, heavier elements gradually build up in the centre of the star. The star forms layers, like an onion, with the heavier elements sat at the centre and the lighter ones towards the outside.

Once the star has built up enough mass (and passes something called the Chandrasekhar limit) the star implodes. The core heats up and becomes even denser before the outer layers are thrown off in a dramatic supernova explosion.

A neutron star is left behind. We'll look at neutron stars next week.

If the original star is 25 times more massive than our Sun, a black hole is left behind. And if the star is more than 100 times more massive than our Sun, nothing is left behind. The star just explodes and leaves nothing behind.

I've oversimplified here as there are three subsets of the type I supernovae: Ia, Ib and Ic. The Ia category supernovae begin as the binary systems we described earlier where the White Dwarf pulls matter from its companion.

Type Ib and Ic supernovae are caused by the implosion of massive stars that have already shed some of their outer layers. Type Ib has lost its hydrogen layer and Type Ic has lost its hydrogen and helium layers.

But supernovae don't like to conform. For example, a very bizarre supernova was recently detected that explodes again and again.

NASA also managed to catch a supernova explosion with the Kepler Space Telescope last year. This animation was produced from the telescope's data as it recorded the supernova:

The other week, I explained how a dying star balloons into a Red Giant when it runs out of fuel. The next stage in this evolution is a White Dwarf.

A Red Giant will eventually blow off the material contained in its outer layers. These expelled outer layers may go on to form planets (more on that later).

All that's left behind is a small and incredibly dense and hot core.

This is a White Dwarf. 

Sirius B (pictured above) is a White Dwarf. Its mass is 98 percent of our own Sun but it is only 12,000 kilometres in diameter, making it smaller than even the Earth and much denser. 

Because they are small and incredibly dense, the gravity on the surface of a typical White Dwarf is around 350,000 times stronger than the gravity on Earth.

No hydrogen fusion occurs to counteract the force of gravity. So, all the matter gets squashed together until all the electrons are pushed far further together than for normal matter.

When all the electrons are squashed together, the matter is now known as a "degenerate" gas. It can't be squashed any further as quantum mechanics dictates there is no more available space for the electrons.​

No. Smaller stars, such as Red Dwarfs, don't reach the Red Giant stage and just burn through all their hydrogen. They should then evolve into White Dwarfs - but Red Dwarfs take trillions of years to consume their hydrogen fuel (which is longer than the age of the universe) so none have gone through this transformation yet.

Massive stars, which are about eight times the mass of our Sun, will never be white dwarfs. Instead, they explode in a violent supernova and either leave behind a black hole or a neutron star. We'll look into the massive supernova explosions that form neutron stars next week...

Many White Dwarfs will fade and eventually become Black Dwarfs when all their energy is radiated away.

However, if a White Dwarf is part of a binary system (where two stars orbit around one another) then its gravitational pull is so large that it may start to pull material off its companion star. ​Adding more mass to the White Dwarf in this way may cause it to become a neutron star or cause a supernova explosion.

If it only pulls off a small amount of matter from the companion star, a smaller explosion called a "nova" may occur. This process can be repeated several times caused a small cosmic fireworks display.

But, if you have two White Dwarfs in a binary system then they may merge together and, again, cause a supernova explosion.

The first exoplanet to be discovered is believed to have been orbiting around a White Dwarf waaaaaay back in 1917. While White Dwarf orbiting exoplanets are highly unlikely to support life, they do provide us with a wealth of information on how to analyse potentially life-harbouring exoplanets.

White Dwarf is also a rather good magazine from British games manufacturer Games Workshop.

And here are five fascinating facts about White Dwarfs and a jolly good video about them:
​

No you smeghead, I'm not talking about the smash Brit comedy Red Dwarf.

Accounting for 80% of the stellar population, Red Dwarfs are the most common type of star in the universe.

​They are smaller than our Sun with between 7.5% and 40% of its mass. Their reduced mass means they have a cooler surface temperature of approximately 3,500K, compared to 5,750K for our Sun. ​A Red Dwarf generates energy in the same way as our Sun, fusing hydrogen into helium.

​Because they are small and cool stars, there's less nuclear fusion and so Red Dwarfs emit only around 10% of the Sun's light - some of the smallest examples have just one ten-thousandth of the Sun's luminosity.

​One of the major differences between Red Dwarfs and other stars is that they're "convective". What this means is that when hydrogen fuses to form helium, the helium does not accumulate in the core. Instead, the helium is continually being mixed throughout the star.

​This means that the nuclear reactions in the star are slowed down and Red Dwarfs have a very long lifetime. It's estimated that a Red Dwarf with a tenth of the Sun's mass will live for at least 10 trillion years. And the lighter a red dwarf is, the longer it lives. ​Because they have such a long life, Red Dwarfs are used to calculate the age of star clusters. 

​Computer modelling suggests Earth-type planets are more likely to form around Red Dwarf stars. One Red Dwarf hit the headlines in February when NASA announced it had seven Earth-sized planets, with two planets orbiting the star in its habitable zone. The TRAPPIST-1 Red Dwarf is 39 light years away from Earth.

Main sequence stars are classified by their temperature. The hottest are blue stars and the coolest are our Red Dwarfs.

Did you know that about 40% of Red Dwarfs host "super earth" planets? Click here to read 10 interesting facts about our universe's most popular stars. Here's a great little video explaining Red Dwarf stars:

And I couldn't live with myself if I didn't include one video from the Red Dwarf show, here are the top 10 highlights from the sci-fi comedy:

Our Sun is shining because its core is fusing hydrogen nuclei together to form helium. It's a main sequence star, which means it's in hydrostatic equilibrium and the outward pressure from this nuclear fusion perfectly balances the gravitational forces trying to collapse the Sun in on itself.

But one day (about 5.4 billion years from now) our Sun will run out of hydrogen fuel in its core and only have helium at its centre. The Sun will stop counteracting gravity and contract slightly. 

When this contraction happens, the temperature will increase - and hydrogen fusion will start to happen in the shell around the helium core. This will cause the Sun to expand.

The outer layers of hydrogen will decrease in temperature, and this makes them look redder.

The Sun is now a red giant.

All main sequence stars between one-fifth and 10 times the mass of our Sun will become red giants when their hydrogen reserves run out in their core.

We're not sure. Scientists believe a red giant Sun will grow large enough to encompass the orbits of Mercury, Venus, and maybe even Earth.

Even if the Earth did survive, its going to be pretty close to the intense heat of the red giant Sun. The Earth's surface will, most likely, be scorched and inhospitable to life.

​The good news is that the Sun's expansion is predicted to alter the Earth's orbit - but the bad news is that we still won't escape a fiery death.

Why do Red Giants expand? You can find out more here. Here's a pretty big repository of red giant facts from Space.com. Red Giants could also explain the origins of the elements in our universe.

​And this is a pretty cool video from Dr. Mark Morris, a professor of astronomy at UCLA, covering the future of our Sun:

Twinkle, twinkle little star. You're not always what you are.

The "stars" you see at night may not be stars. Some of them are planets. Some of them are galaxies. But most of them are stars - and they're all completely unique.

But the human race does like to put things into boxes, so here are all the categories of early-life stars*:

A protostar is what you have before a star forms. It's the swirling collection of gas that's collapsing down into a star. It's getting hotter and denser - but nuclear fusion reactions haven't started yet.

Just before a star moves onto the main sequence, they're classified as a T Tauri. They look like a main sequence star and have the same temperature, but they're brighter because they're larger. The key difference is that a T Tauri generates its energy by gravitational pressure - not nuclear fusion.

The majority of the stars in our galaxy are main sequence stars. They're in a state called hydrostatic equilibrium. This means the gravity of the star that's pulling everything together is balanced by the light pressure from the nuclear fusion reactions pushing everything outwards. The result is that the star is spherical (or nearly spherical) in shape.

Main sequence stars are a pretty diverse bunch and can change in colour, size, brightness and mass but they all do the same thing: convert hydrogen to helium in their cores, a process called nuclear fusion, which releases a tremendous amount of energy.

Main sequence stars are classified by their luminosity (aka their brightness) into seven categories: O, B, A, F, G, K and M. O are the hottest and blue in colour, M are the coolest and read in colour.

Main sequence stars can be as small as 8% the mass of our Sun and can, theoretically, go up to 100 times the mass of the Sun.

Smaller bodies (less than 8% the mass of our Sun) do not have enough mass for nuclear fusion reactions to start. These brown dwarfs are too small to be stars and too big to be planets - and they never twinkle.

Next week, I'll look at what happens at the end of a star's life...

This is a pretty thorough description of the different classifications of main sequence star. Here's some more information on brown dwarfs.

This post goes into a little more depth around main sequence stars and here's some more information on the nuclear fusion reactions in stars that keep them shining.

* By early-life I mean those stars that are about to, may never, or have achieved the nuclear fusion of hydrogen to helium.

​Hello. I’m the freelance writer who gets tech. I have two degrees in Physics and, during my studies, I became increasingly frustrated with the complicated language used to describe some outstanding scientific principles. Language should aid our understanding — in science, it often feels like a barrier.
​
So, I want to simplify these science sayings and this blog series “Sunday Science” gives a quick, no-nonsense definition of the complex-sounding scientific terms you often hear, but may not completely understand. 

If there’s a scientific term or topic you’d like me to tackle in my next post, fire an email to [email protected] or leave a comment below. If you want to sign up to our weekly newsletter, click here.

Rockets are used to transport objects and people into space. That's a really dull sentence to describe potentially the most EXCITING engineering achievement of the human race.

But how do they work? Well, there are four basic forces of flight: lift, gravity, thrust and drag. Thrust and lift are positive forces that propel a rocket into space. Gravity and drag are negative forces that slow a rocket down.

It's a bit like letting air out of a balloon. The weight of the balloon tries to pull it down to Earth (gravity) and when it's flying around the room, air resistance (drag) pulls the balloon in the opposite direction of its movement.

The air whooshing out of the balloon provides the thrust it needs to oppose its weight. The lift is a force at right angles to the thrust, which stabilises and controls the direction of flight. 

Rockets don't have air. They carry a lot of fuel to get them into space. When this liquid fuel burns, it produces gas. The build up of this exhaust gas escapes the rocket with a lot of force and provides enough thrust for the rocket to blast off.

Rockets need a huge amount of energy to overcome gravity and stop them falling back to Earth. In his famous 1962 speech championing travel to the Moon, US President John F. Kennedy compared the power of a rocket to "10,000 automobiles with their accelerators on the floor." 

To give you a more exact figure, a rocket needs to achieve speeds of 25,000 mph to escape the Earth's gravity. 

Modern rockets are made up of several parts. The main parts are the propulsion system (to get the rocket into space), payload system (where the astronauts sit) and guidance system (to manoeuvre the rocket), but they can contain around three million different parts. ​They all look different too, but here's a basic picture of a rocket, courtesy of NASA:

This is a more thorough (but not overly complicated) explanation on how rockets work and here's some more information on their structure. If you want to know a little bit more about how Ironman flies, check out this awesome explanation.

I've distilled a really interesting topic into a short post. For example, there's been a lot written about the space race between the Russians and Americans, but I'd hardheartedly recommend you read Hidden Figures: The Story of the African-American Women Who Helped Win The Space Race. Also, did you know that the first true rocket was used in 1232? The history of the rocket is a fascinating tale that goes beyond the space race - you can read more here and here. Also, check out this in-depth explanation of the final stages before blast off!

A rocket launch is a pretty spectacular sight. Speaking last week, Tim Peake said: “I was mesmerised by the noise and the power of the rocket launch when I first saw one.” So, here's a launch in action - make sure you turn up the volume to get an idea of how LOUD one can be!

​Could we colonise Mars by 2024? Elon Musk seems to think so and astronaut Tim Peake predicted a similar timescale for a trip to the Red Planet when he spoke at York University, a couple of days before Musk's prediction was made.

Echoing Apollo astronaut Charlie Duke's optimism around a manned mission to Mars, Peake predicts we will see humans on Mars by the late 2030s "but no one nation has the resources to do this. If we can include private investment and companies like SpaceX this could speed things up," he added.

Peake was a little more considered in his predictions than Musk. "People are always over ambitious when talking about space exploration. It is hard and difficult work," he said.

"One would like to think we would have permanent occupancy modules on Mars in 100 years," according to Peake.

Tim Peake shared a lot of nuggets of information about space exploration, living in space and the importance of snorkels and nappies during a space walk (yes, really). Here are my 12 favourite titbits from his fascinating talk:

Peake flew from Russia’s Baikonur Cosmodrome in Kazakhstan, where a whole other set of prelaunch traditions are in place.

Most of these rituals pay tribute to the first human to go into space, Yuri Gagarin, including peeing on the back tyre of the bus that takes cosmonauts to the launch pad, just as Gagarin did in 1961. According to Tim, a space suit is not the easiest item of clothing to undo and do up again.

​Tim traveled to the Space Station with ​Veteran Russian commander Yuri Malenchenko and NASA astronaut Tim Kopra in the Soyuz capsule.

The Soyuz capsule had to attach to the Space Station but the automatic docking system failed to operate, so manual control had to be taken.

It sounded like a pretty hairy time as the optics on the capsule were flooded with light as the transition from day to night occurred when the automatic docking system failed.

Malenchenko took control and eventually docked the craft. "He knew the danger and waited until better conditions," Peake added.

When Soyuz's crew met the crew at the Space Station, their physical and mental condition was "great", according to Tim, despite spending nine-months in space.

The working week on the Space Station is also your (not so typical) 9 to 5. So, it is structured and, at times, quite solitary according to Peake as each crew member works on different scientific experiments.

“No two days are the same and that's what makes it so exciting," according to Peake, who also took remote control of a Mars rover (which was based on Earth at the time) while on the Space Station. It was an important proof of concept for future exploratory missions to the Red Planet.

Tim and the crew also had to clean the Space Station every Saturday (do astronauts have feather dusters? I'd like to think so). Tim also said the station had a "noisy party atmosphere" because of all the different instruments left to run, which mean there's a constant 40-50 decibels of noise.

"It also smelt of a laboratory," Tim added. But you soon get used to the smell.

Tim conducted one space walk during his time on the Space Station to fix a faulty electrical box. Peake said this was "the moment of greatest apprehension because you do not know what that moment will feel like."

Dropping out of the airlock, Peake said his primary concern was getting tangled in the tethers on his suit and "the second thing was not to look down as it's an awfully long way down at 400km above the Earth."

The space walk was running 10 minutes ahead of schedule, so Tim had to wait for the Sun to set and the power supply to be cut before the repair work to the electrical box could be carried out. "Mission control said hang out. I don't think anyone else has been told to hang out in space. It was one of the most remarkable moments of my life," according to Peake.

"I felt myself completely immersed in space and had a quiet reflection on where we were and what we were doing."

The CO2 sensor for Tim's fellow spacewalker, Tim Kopra, went off during the excursion, which usually signals a water leak in the suit.

"We have a procedure to deal with this," according to Peake, "A snorkel so you can still breathe and a nappy at the back of the helmet to absorb moisture."

All that scientific knowledge and development and "we came up with a snorkel and a nappy," Tim added.

The spacewalk was terminated early as a result, but the astronauts were outside the Space Station for 4 hours and 43 minutes.

When Tim did have a spare few minutes (he also completed a huge amount of outreach work), he photographed the mind-blowing views from the Space Station. Check out his stunning book of those views (plus, all Tim's proceeds go to the Prince's Trust).

"Watching Earth from space is mesmerising and it's constantly changing. During the day, you do not see borders or cities, it's all about the mountains and glaciers. Earth is a wonderful geological feature in the making. At night, the Earth comes alive," Peake added.

Reentry sounds like quite a traumatic experience. Tim described the experience as if "the capsule is blowing itself apart."

And, when the breaking parachutes slowly open, "it was 20 seconds of the most crazy roller coaster ride of my life," according to Peake. "You don't want to have your tongue between your teeth. You're bracing for something that is essentially a car crash," he added. Yikes.

Readjusting to life on Earth isn't easy. Peake found he was dizzy, disoriented and felt pretty rough for the first three days on Earth. Muscle distribution is something that really suffers, according to Peake, and he is only just starting to fully recover his bone density.

I don't know about you, but the thought of going into space terrifies me. Peake, on the other hand, was quietly cautious when talking about the launch process.

"You deal with the consequences of what you are doing long before you go on the rocket," he said. "No one should happily sit on 300 tonnes of rocket fuel - if you can then you may not be psychologically prepared to go to space."

Peake's time on the Space Station has changed his perspective on our planet. "You feel like you know the Earth pretty well, even though there are areas you have never been to, or are likely to visit."

This "overview effect" is a psychological phenomenon shared by many astronauts.

While there's no gravity on the Space Station, there is a convention of what's up and what's down. It's also important to be a "good crew member" when it comes to personal hygiene on the Space Station. Although your time there means "it's the best pedicure you'll ever have", you have to be careful when you whip your socks off, according to Peake.

And the astronauts returned with "lizard feet" as they hooked their feet to steady themselves as they moved around in zero gravity, which meant they rubbed in unusual places.

It looks like Tim will have to cope with lizard feet at least one more time too, as he's due to go back up to the Space Station. And he also has high hopes for his future in space, as he added:

With only six moonwalkers now left on Earth, fingers crossed for Tim - and the future of space exploration, as we know it.

SETI is a rather simple acronym that deals with one of humanity's biggest questions: are we alone in the universe?

The Search for Extraterrestrial Intelligence (SETI) is looking for advanced civilisations. It's not interested in tiny microbes or any such simplistic lifeforms.

Most SETI searches hunt for radio and optical signals to indicate intelligent life. 

Radio astronomy is used to hunt for radio signals that an intelligent civilisation could produce. These tend to look for narrow-band signals, which are radio emissions that only cover a tiny part of the radio spectrum. This is because natural objects will emit radio waves across the spectrum, but if you find a signal that just uses a small region of the radio spectrum, it could have an artificial source. Scientists also use optical searches to look for brief flashes of light in the search for intelligent life.

​Such optical and radio signals could be deliberately beamed out of a planet, or they could be picked up accidentally. Earth has unintentionally broadcast radio and radar signals since World War 2. We also purposefully transmitted a simple message from the Arecibo Observatory in Puerto Rico in 1974.

This radio message (shown above) carried basic information about the human race and was fired at the globular star cluster M13 in the hope that extraterrestrial intelligence might receive and decipher it. 

The SETI Institute is the largest player in the hunt for advanced civilisations in our cosmos.

Set up in 1988, it is now a not-for-profit organisation (after its funding was withdrawn after a year) and is made up of scientists, engineers, teachers and other staff.

The Institute has more than 100 active projects. And, in a joint project with the University of California, Berkeley it built the Allen Telescope Array - a 42-strong radio telescope array to examine one million stars in the next two decades.

Despite hopes being raised just over a year ago, SETI hasn't found any evidence of intelligent extraterrestrial life.

We'll just have to wait a little bit longer for ET to phone home.

There's so much information on the SETI Institute's website and the FAQ section is particularly useful. 

You can also analyse the light from an exoplanet (a planet outside of our own Solar System) to work out what its atmosphere is made up of. If the atmosphere is made up of oxygen, nitrous oxide and methane, then this could indicate life is present. If you'd like to find out more about exoplanets - NASA's Kepler space telescope has found dozens of potentially habitable worlds, including this mysterious "alien megastructure".

And here's a list of the seven most likely places in the universe where intelligent life could exist.

​Want more Sunday Science? I've just started the Sunday Science Facebook Group. If you've got a question about science, want to see more science on your news feed or just want to keep up to date with the latest on the blog, it would be great to welcome you to the group!

​Hello. I’m the freelance writer who gets tech. I have two degrees in Physics and, during my studies, I became increasingly frustrated with the complicated language used to describe some outstanding scientific principles. Language should aid our understanding — in science, it often feels like a barrier.
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So, I want to simplify these science sayings and this blog series “Sunday Science” gives a quick, no-nonsense definition of the complex-sounding scientific terms you often hear, but may not completely understand. 

If there’s a scientific term or topic you’d like me to tackle in my next post, fire an email to [email protected] or leave a comment below. If you want to sign up to our weekly newsletter, click here.