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## First ever asteroid from another solar system detected

In late October 2017, astronomers announced the first ever discovery of an asteroid (or comet?) coming into our Solar System from another stellar system. The object was first spotted on 19 October by the University of Hawaii’s Pan-STARRS telescope, during its nightly search for near-earth objects. Based on its extreme orbit and its rapid speed, it was soon determined that the object has come into our Solar System from somewhere else, and this makes it the first ever asteroid/comet with an extra-solar origin to have been discovered. Originally given the designation A/2017 U1, the International Astronomical Union (IAU) have now renamed it 1I/2017 U1, with the I standing for “interstellar”.

The object, given the designation A/2017 U1, was deemed to be extra-solar in origin from an analysis of its motion.

In addition to its strange trajectory, observations suggest that the object also has quite an unusual shape. It is very elongated, being ten times longer than it is wide. It is thought to be at least 400 metres long but only about 40 metres wide. This was determined by the rapid and dramatic changes in its brightness, which can only be explained by an elongated object tumbling rapidly.

The object has also been given the name Oumuamua (pronounced oh MOO-uh MOO-uh), although this is not its official name (yet).  This means “a messenger from afar arriving first” in Hawaiian. In other respects, it seems to be very much like asteroids found in our own Solar System, and is the confirmation of what astronomers have long suspected, that small objects which formed around other stars can end up wandering through space, not attached to any particular stellar system.

## The origin of the elements

A couple of weeks ago this fascinating version of the periodic table of the elements was the NASA Astronomy Picture of the Day (APOD). Most people have seen the periodic table of the elements, it is shown on the wall of most high school chemistry classrooms. But, what is totally fascinating to me about this version is it shows the origin of each element.

It has been a long process of several decades to understand the origin of the elements. In fact, we have not totally finished understanding the processes yet. But, we do know the story for most elements. All the hydrogen in the Universe was formed in the big bang. This is true for nearly all the helium too. A small amount of the 25% or so of helium in the Universe has been created within stars through the conversion of hydrogen into helium. But, not much has been created this way because most of that helium is further converted to carbon.

The only other element to be formed in the big bang is lithium. About 20% of the lithium in the Universe was formed in the big bang, the rest has been formed since,

Together, hydrogen and helium comprise 99% of the elements in the Universe by number (not by mass).

Where Your Elements Came From – from the NASA Astronomy Picture Of the Day (APOD) 24 October 2017.

I have decided to use this fascinating table as the basis for a series of blogs over the next few weeks to explain each of the 6 processes in these six boxes

## “Astrophotography” now available

My latest book, Astrophotography, is now available. You can order a copy by following this link. Astrophotography is a book of exquisite images of space, including some of the latest images such as New Horizons’ images of Pluto, Rosetta’s images of Comet 67P, and Hubble Space Telescope images of the most distant galaxies ever seen. Each stunning image, reproduced to the highest quality, is accompanied by text that I have written to explain the object, and any background science relating to the object.

Astrophotography is now available. You can order your copy by following this link.

One unique aspect of Astrophotography is that it emphasises the multi-wavelength approach taken to understanding astronomical objects. For millennia we could only study the Universe in visible-light (the light to which are eyes are sensitive), but for the last few decades we have used every part of the electromagnetic spectrum from radio waves to gamma rays to better understand the Universe. This multi-wavelength approach has also enabled us to discover previously unknown aspects of the Universe such as the Cosmic Microwave Background, the true appearance of Venus’ surface which lies hidden below its thick atmosphere, and huge quantities of gas between galaxies (the intracluster medium) which emit no visible-light but prodigious amounts of X-rays.

Astrophotography is split into 5 sections, namely

1. Exploring the Solar System
2. Exploring the Milky Way
3. Exploring the Local Group
4. Beyond the Local Group
5. At the Edge of the Universe

Below are examples of some of the beautiful images found in Astrophotography, along with examples of the accompanying text. At the beginning of each page’s text I caption which telescope or space probe has taken the main image, and at which wavelength (or wavelengths).

## Exploring the Solar System

Two examples from the first section of Astrophotography, the section on the Solar System, are stunning images of Mercury and of Mars. The images of Mercury were taken by NASA’s MESSENGER spacecraft. There are several pages of images of Mars, the page shown below shows an image of the Martian surface taken by the Mars Curiosity Rover, and an image of Victoria Crater taken by the Mars Reconnaissance Orbiter.

Images of Mercury taken by NASA’s MESSENGER spacecraft. The four main images are spectral scans, and show information on the chemical composition of Mercury’s surface.

The section on the Solar System also includes images of Pluto taken by New Horizons, images of Saturn and Titan taken by the Cassini space probe, images of Comet 67P taken by Rosetta, and images of Jupiter and her moons taken by the Galileo space craft.

The surface of Mars as imaged by NASA’s Mars Curiosity Rover and, at right, Victoria Crater, as imaged by NASA’s Mars Reconnaissance Orbiter.

## Exploring the Milky Way

The second section of Astrophotography includes images of the Orion Nebula (Messier 42), the reflection nebula Messier 78, the Horsehead Nebula, the Pillars of Creation (part of the Eagle Nebula), and the Crab Nebula, the remnant of a supernova which exploded in 1054.

The example I show below is of the reflection nebula Messier 78, and is a visible light image taken by the Max Planck Gerzellschaft Telescope, a 2.2 metre telescope located at the European Southern Observatory’s facility in La Silla, Chile. The text describes the history of observing Messier 78, and explains what produces a reflection nebula.

The reflection nebula Messier 78 imaged in visible light by the Max Planck Gesellschaft Telescope. The text explains what reflection nebulae are, and the history of observing this particular object.

## Exploring the Local Group

The third section of Astrophotography looks at the Local Group, our part of the Universe. The Local Group includes our Milky Way galaxy, the Large and Small Magellanic Clouds, and the Andromeda galaxy. Some of the images shown in this section include the Tarantula Nebula in the Large Magellanic Cloud, NGC 602 (in the Small Magellanic Cloud), the Andromeda galaxy, Supernova 1987A and the Seahorse Nebula.

The example I show here is the Seahorse Nebula, a dark cloud of gas and dust located in Large Magellanic Cloud. This Hubble Space Telescope image was taken in 2008, and the nebula is in the bottom right of the image.

The Seahorse nebula is a dark cloud of gas and dust found in the Large Magellanic Cloud, an irregular galaxy visible to the naked eye and in orbit about our Milky Way galaxy. The seahorse nebula is in the bottom right of the image.

## Beyond the Local Group

The fourth section of Astrophotography looks at the rich variety of galaxies found beyond our own neighbourhood. Examples are galaxies like Messier 82, which is undergoing a huge burst of star formation in its centre, Centaurus A, which shows huge lobes of radio radiation stretching far beyond the stars we see in visible light, colliding galaxies such as The Antennae galaxies, and evidence for dark matter such as the Bullet cluster.

The example I have shown here is the spread for Messier 81, a beautiful spiral galaxy found in Ursa Major. It is one of the best known galaxies in the sky, and is visible to northern hemisphere observers throughout the  year. The main image illustrates the multi-wavelength approach astronomers take to studying many objects. The image combines visible light, infrared light and ultraviolet light to teach us far more about the galaxy than we would learn if we only looked in visible light.

Messier 81 is a beautiful spiral galaxy found in Ursa Major. Hence it is visible throughout the year to northern hemisphere observers. The main image shown here is a combination of of a visible light image (taken by the Hubble Space Telescope), an infrared image taken by the Spitzer Space Telescope, and an ultraviolet image taken by Galaxy Evolution Explorer (GALEX).

## At the edge of the Universe

In the final section of Astrophotography, I show examples of some of the most distant objects known. Images include the Hubble Deep Field, the Cosmic Microwave Background, the most distant galaxy seen (GN-z11, lying about 13.4 billion light years away), gravitational lenses and the recent discovery of gravitational waves made by LIGO.

The example I show here is the spread about the gravitational lens SDP81, a galaxy lying about 12 billion light years away which is being lensed (and brightened) by an intervening cluster of galaxies which lie about 4 billion light years away. The top image was taken at millimetre wavelengths by the Atacama Large Millimetre Array (ALMA), the bottom image in visible light by the Hubble Space Telescope.

Gravitational lenses enable us to see distant galaxies which would otherwise be too faint to see, but they also provide us with a way of tracing the distribution of dark matter in clusters.

I hope these few examples from Astrophotography have whetted your appetite to find out more. I really enjoyed putting the book together, and am very pleased with the quality of the images and their aesthetic beauty.

## Available in all good bookshops – soon!

Some of you may have noticed that I haven’t blogged much this last month. The reason is that I have been putting the finishing touches on a book – which has just been sent off to the publishers Springer. I am sure it will need some revision, but am also hopeful that it should be hitting the shelves / bookshops / electronic stores in the next few months.

The cover, even the title may change!

## Derivation of Newton’s form of Kepler’s 3rd law – part 1

In 1619, Johannes Kepler published a relationship between how long a planet takes to orbit the Sun and the size of that orbit, something we now call his 3rd law of planetary motion, or just “Kepler’s 3rd law”. It states that

$T^{2} \propto a^{3}$

where $T$ is the period of the orbit and $a$ is the size of the orbit. Kepler also found that the planets orbit the Sun in elliptical orbits (his 1st law), and so the size of the orbit $a$ that we refer to is actually something called the “semi-major axis”, half the length of the long axis of an ellipse.

Any proportionality can be written as an equality if we introduce a constant, so we can write

$T^{2} = k a^{3} \text{ (equation 1)}$

where $k$ is our constant of proportionality.

Kepler found that the planets orbit the Sun in ellipses, with the Sun at one of the foci. The long axis of an ellipse is called its major axis. The $a$ in Kepler’s 3rd law refers to the length of the semi-major axis of a planet’s ellipse.

Newton was able to show in his Principia, published in 1687, that this law comes about as a natural consequence of his laws of motion and his law of gravity. How can this be shown?

## Why is Kepler’s law true?

To show how Kepler’s law comes from Newton’s laws of motion and his law of gravitation, we will first of all make two simplifying assumptions, to make the mathematics easier. First we will assume that the orbits are circular, rather than elliptical. Secondly, we will assume that the Sun is at the centre of a planet’s circular orbit. Neither of these assumptions is strictly true, but they will make the derivation much simpler.

Newton’s law of gravity states that the gravitational force between two bodies of masses $M \text{ and } m$ is given by

$F = \frac { G M m }{ r^{2} } \text{ (equation 2)}$

where $r$ is the distance between the two bodies and $G$ is a constant, known as Newton’s universal gravitational constant, usually called “big G”. In the case we are considering here, $r$ is of course the radius of a planet’s circular orbit about the Sun.

When an object moves in a circle, even at a constant speed, it experiences an acceleration. This is because the velocity is always changing, as the direction of the velocity vector is always changing, even if its size is constant. From Newton’s 2nd law, $F=ma$, which means if there is an acceleration there must be a force causing it, and for circular motion this force is known as the centripetal force. It is given by

$F = \frac{ m v^{2} }{ r } \text{ (equation 3)}$

where $m$ is the mass of the moving body, $v$ is its speed, and $r$ is the radius of the circular orbit. This centripetal force in this case is provided by gravity, so we can say that

$\frac{ G M m }{ r^{2} } = \frac{ m v^{2} }{ r }$

With a little bit of cancelling out we get

$\frac{ G M }{ r } = v^{2} \text{ (equation 4)}$

But the speed $v$ is given by the distance the body moves divided by the time it takes. For one full circle this is just

$v = \frac{ 2 \pi r }{ T }$

where $2 \pi r$ is the circumference of a circle and $T$ is the time it takes to complete one full orbit, its period. Substituting this into equation (4) gives

$\frac{ G M }{ r } = \frac{ 4 \pi^{2} r^{2} }{ T^{2} }$

Doing some re-arranging this gives

$\boxed{ T^{2} = \frac{4 \pi^{2} }{ GM } a^{3} } \text{ (equation 5)}$

where we have substituted $a$ for $r$. This, as you can see, is just Kepler’s 3rd law, with the constant of proportionality $k$ found to be $(4 \pi^{2})/(GM)$. So, Kepler’s 3rd law can be derived from Newton’s laws of motion and his law of gravity. The value of $k$ above is true if we express $a$ in metres and $T$ in seconds. But, if we express $a$ in Astronomical Units and $T$ in Earth years, then $k$ actually comes out to be 1!

## Newton’s form of Kepler’s 3rd law

A web search for Newton’s form of Kepler’s 3rd law will turn up the following equation

$(M + m) T^{2} = a^{3} \text{ (equation 6)}$

How can we derive this? I will show how it is done in part 2 of this blog, as we will need to learn about something called “reduced mass”, and also the “centre of mass”.

I mentioned in this blog here that I would be on TV talking about the calculation that the Milky Way galaxy contains some 17 billion Earth-like planets.

Here is a youtube video capture of my appearance on the TV show. My apologies that the subtitles lag behind what is being said, and for the subtitles only being a summary of what is said. But at least if will give you a vague idea of what I’m saying if you cannot understand Welsh.

## Humanity’s most distant travellers (part 1)

Next Thursday (27th of September), I am going to be on live TV talking about the Voyager space probes. These remarkable crafts left our small planet in 1977, and are now on the point of entering interstellar space. Voyager 1 is currently 18.2 billion km from the Sun, and Voyager 2 is 14.8 billion km away. They are, by a large margin, the most distant objects human beings have ever sent into space.

This cartoon, from the official NASA Voyager website, shows where the two spacecraft are, compared to the heliopause. The heliopause is a theoretical boundary where the Sun’s Solar wind is stopped by the stellar winds from other, nearby stars. The heliosphere is the sphere of space within which the Sun and the planets reside, as they are within the heliopause, the surface of this sphere (in reality it is not a sphere, it would only be a sphere if the radiation from nearby stars was perfectly uniformly incident on our Solar system).

This cartoon, from the NASA Voyager website, shows the position of the two Voyager space probes, and the heliosphere and heliopause.

Because these distances are so huge, it is sometimes easier to use larger units. The Astronomical Unit (AU) is defined as the average distance from the Earth to the Sun, so 150 million km. In these units, Voyager 1 is 122 AU from the Sun, and Voyager 2 is 99 AU away. The weak signals that we are still able to receive from the two space craft travel at the speed of light, and are currently taking about 30 hours (not minutes as I previously typed) to get to us.

The Voyager space probes were identical copies of each other, but were launched a few weeks apart and went on a different journey into the outer Solar System

## Voyager 1 and 2 at Jupiter

Voyager 1 arrived at Jupiter in January 1979. Voyager 2 reached the planet in July of the same year. Both space craft returned the most detailed pictures yet of the Solar System’s largest planet. In addition to making important studies of the great red spot, the two probes made the surprising discovery of volcanic activity on Io, Jupiter’s closest moon.

A Voyager 1 image of Jupiter’s great red spot

The plume of material on the left is a volcanic eruption on the moon Io, the nearest of the Galilean moons.

We now know that Io is the most volcanically active body in the Solar System. The source of its internal heating is the tidal forces from Jupiter. Because it doesn’t orbit Jupiter in a perfect circle, but rather in an ellipse, the Moon gets repeatedly deformed in different directions and this heats its interior up (in the same way that squeezing a tennis ball repeatedly will lead to its getting warm). With a warm molten interior, the conditions are just right for this to escape through the crust as volcanic eruptions.

## Voyager at Saturn

In November 1980, Voyager 1 flew past Saturn. By August 1981, Voyager 2 had arrived at the ringed planet. Voyager made important discoveries about Saturn and her moons, in particular about Saturn’s rings. It discovered new ring structures, and even “spokes” in the rings.

A backlit image of Saturn taken by Voyager 1 after its flyby in late 1980.

Voyager 2 discovered mysterious spokes in Saturn’s rings. It was many years before we understood what causes these.

Later this week I will write a part 2 to this blog, talking about Voyager 2’s encounters with Uranus and Neptune, the famous pale blue dot photograph, and the messages being carried on the probes as they head off into interstellar space.

Here are part 2 and part 3 of this post.