How Big Is A Neutron Star?

Neutron Star

Size of a Neutron Star

One of the wonders of the cosmos is the neutron star, a star in which the atoms have collapsed. In a single cubic centimeter of neutron star material – a chunk little more than the size of a sugar cube – there are are around 100 million tons of matter. An entire neutron star occupies a sphere that is roughly the size across the island of Manhattan (about 14 miles).

Formation of a Neutron Star

A neutron star is the collapsed core of a giant star. Neutron stars are the smallest and densest stars, not counting black holesquark stars and strange stars.

Neutron stars result from the supernova explosion of a massive star, combined with gravitational collapse, that compresses the core past white dwarf star density to that of atomic nuclei. Once formed, they no longer actively generate heat, and cool over time.

There are thought to be around 100 million neutron stars in the Milky Way, a figure obtained by estimating the number of stars that have undergone supernova explosions.

The Universe Inside You
The Universe Inside You: The Extreme Science of the Human Body

Brian Clegg

Background on Brian Clegg

How Did The Days Of The Week Get Their Names?

Sun and Moon

Weekly Seven-Day Cycles

The idea of dividing the cycle of the moon into four seven-day weeks may have begun in Babylon. In its familiar modern form, it probably derives from a Jewish model, echoing the story of Creation as told in Genesis, in which God, having made the world in six days, rested on the seventh – and ordered humanity and their animals to do likewise. As a result, every week connects us to the beginning of time itself, as the days plot the round of our work and leisure, the recurrent rhythm of our existence.


Our Language and Beliefs

The weekday names depend on our language and our beliefs. The names that we give the weekdays in English are an inherited meditation on the cycles of time, as we observe the pattern of the sun, the moon and the planets circling above us – though the story they tell us is for English-speakers only, since nobody else’s week is quite the same as ours.

Days Named After Gods

Sunday, Monday – the week begins with the sun and the moon, whose separate movements mark the months and years. After them, come the days of the easily visible planets. In Romance languages, this is Mars, Mercury, Jupiter, and Venus – the sequence that the Romans followed and left behind.


In England, around the seventh century, the planets tethered to the gods of Rome were renamed for the equivalent northern gods, and it is their Anglo-Saxon names – Tiw, Woden, Thor, and Frige – that distinguish the days for English-speakers on Tuesday, Wednesday, Thursday, and Friday. On Saturday, the Anglo-Saxon gods are joined by Saturn, which retained its Latin name, making our week, like our language itself, a peculiar German-Latin hybrid.

Cosmological History

Encompassing the different cycles of sun, moon and the five planets, every week thus implies not just a long span of many years, but also the company of gods and the vastness of space itself. In the names of our days is the entire solar system – the time-space continuum as it was known in the ancient Mediterranean world and transmitted to the north of Europe. The turn of the week is – in English – a concise cosmological history, in which we still live every day with the gods of our ancestors inhabiting an ancient, but stable structure of time.

Living With the Gods
Living With the Gods: On Beliefs and Peoples

Neil MacGregor

Background on Neil MacGregor

How Many Atoms Are In The Human Body?

Inside Human Body

Looking Into a Mirror

Standing in front of a full-length mirror, you can look at your whole body, viewing it as a single remarkable object – a living creature. But you can also dig into details, exploring the ways your body interacts with the world around it, or how it makes use of the energy in the food you eat to get you moving.

Number of Cells in a Body

Zoom in further and you will find somewhere between 10-100 trillion cells. Each cell is a sophisticated package of life, yet taken alone, a cell certainly isn’t you. Go further still and you will find complex chemistry abounding – you have a copy of the largest known molecule in most of your body’s cells: the DNA  in chromosome 1.

Number of Atoms in a Body

Continue to look in even greater detail and eventually you will reach the atoms that make up all matter. Here traditional numbers become clumsy – a typical adult is made up of around 7, 000, 000, 000, 000, 000, 000, 000, 000, 000 atoms. It’s much easier to use scientific notation to denote the number of atoms in a human body: 7 x 10 27 atoms, meaning 7 with 27 zeroes after it.

The Universe Inside You
The Universe Inside You: The Extreme Science of the Human Body

Brian Clegg



Brian Clegg
Brian Clegg
born 1955

. Brian Clegg website
. Wikipedia

Brian Clegg is an English science writer. Clegg is the author of popular science books on topics including light, infinity, quantum entanglement, and surviving the impact of climate change.

Books Brian Clegg has written include:

Is Optimism a Way of Creating the Future?


We Are Responsible for the Future

The possibilities that lie in the future are infinite. When I say ‘It is our duty to remain optimists,’ this includes not only the openness of the future, but also that which all of us contribute to it by everything we do.

We are responsible for what the future holds in store. It is our duty, not to prophesy evil, but, rather, to fight for a better world..

Karl Popper, The Myth of the Framework: In Defense of Science and Rationality

The Principle of Optimism

All evils are caused by insufficient knowledge.

Optimism is a way of explaining failure, not prophesying success. It says that there is no fundamental barrier, no law of nature or supernatural decree, preventing progress.

Whenever we try to improve things and fail, it is not because the spiteful (or benevolent) gods are thwarting us or punishing us for trying, or because we have reached a limit on the capacity of reason to make improvements, or because it is best that we fail, but always because we did not know enough, in time.

In addition, optimism is a stance towards the future, because nearly all failures, and nearly all successes, are yet to come.

The Beginning of Infinity
The Beginning of Infinity: Explanations That Transform the World

David Deutsch

Background on David Deutsch

How Did Ancient People Use Calendars?

Oldest Calendars

The oldest ‘calendars‘ are vast archaeological sites that aligned posts or megaliths (giant stones) with the rising of the Sun or Moon on significant dates, such as the summer or winter solsticeThe earliest structures built as calendars seem to be designed to help calculate the solar and lunar months. Priest-astronomers used these. 

Prehistoric astronomers left no user manuals for their monuments – their uses had to be rediscovered by archaeoastronomersarchaeologists with knowledge of astronomy. 

Warren Field, Scotland, found in 2004, is the earliest site found so far. It tracks events some 10,000 years ago.

Development of Accurate Calendars

Calendar development was often driven by the need to fix religious festivals and observance, an impulse that continued with the formation of new religions such as Christianity and Islam. Both put astronomy to use in this way.

Arab astronomers and engineers were zealous in their pursuit of improved methods for keeping time so that daily prayers could be received by the devout at the right time.

Time-keeping on a larger scale was essential in scheduling religious festivals.

Lunar Calendars 

Time is naturally divided astronomically by the Earth’s orbit around the Sun (a year), the Earth’s rotation (a day) and the phases of the Moon. A lunar month (a full cycle from one new or full moon to the next is approximately 29 1/2 days. A year is 365 1/4 days.

Inconveniently, a year is 12.37 lunar months long. For early societies, a lunar month was a useful and countable period of time, one that could be easily observed and checked just by looking up at the night sky.

But if you use 12  lunar months as the basis of your year, the calendar will drift out of sync quite quickly. It will be a month out after only 3 years, and 6 months out after 18 years. To avoid this, an extra (intercalary) month has to be added every few years.

Egyptian Calendars

The ancient Egyptians began their year with the rising of Sirius (which they called Sopdet) above the horizon before sun rise.

  • System dates to c. 3000 BC.
  • Divided year into 365 days.
  • Used two different calendars.
  • Sirius, a stable star, and the brightest star in the night sky, was the basis for the Egyptian calendar.
  • Ptolemaic rulers depended on calendars.

Other cultures developed independent calendars, notably China and Mesoamerican (Central American) civilizations.

Chinese Yin-Yang Li Traditional Calendar

Origins of the Chinese calendar can be traced back to 14th century BC, though legend says it was invented in 2637 BC.

  • Literally ‘heaven-Earth’ calendar.
  • Used alongside imported calendars such as the Hindu calendar.
  • Used until 1912, when China officially adopted the Western Gregorian calendar.

How the World Works: Astronomy: From Plotting the Stars to Pulsars and Black Holes
How the World Works: Astronomy: From Plotting the Stars to Pulsars and Black Holes 

Anne Rooney

Background on Anne Rooney