Sirius

As most people with an interest in astronomy know, Sirius is the brightest star in the night sky – easily outshining all other stars.

The apparent brightness of a star depends on two factors: how luminous it is (its intrinsic brightness) and how far away it is. Sirius is a bright star, 25.5 times more luminous than the Sun,  and in astronomical terms it is close. It lies at a distance of  8.6 light years, in celestial terms this is our back yard, making it the seventh nearest star to the Sun.  

Sirius lies in the constellation Canis Major, the Great Dog, which lies to the southeast of Orion. For this reason, it is also known as The Dog Star.

Origin of the term Dog Days

Sirius lies in the southern half of the sky. At all locations in the Northern Hemisphere where it can be seen it will only be above the horizon for part of the day.

 Anyone who has carefully observed the night sky over a period of time may notice that stars such as Sirius which aren’t circumpolar (which means they never set) rise about four minutes earlier each day.  The rising and setting times  depend upon the observer’s latitude and, because of the four-minute daily shift, the time of year. If we take the example of Rome which lies at a latitude of 42 degrees north, then rising and setting times for the Sun and Sirius over the course of the year are as follows.

Sources https://www.timeanddate.com/sun & https://codebox.net/pages/star-rise-and-set-time-calculator

Between early May and early August Sirius is only above the horizon during daylight hours and thus cannot be seen against the brightness of the daytime sky. On 3 August Sirius and the Sun rise at the same time and shortly afterwards Sirius becomes visible in the morning twilight before the Sun has risen. By 1 September Sirius rises a full two and a half hours before the Sun.

To the ancient Romans, living in an era before electric lighting and light pollution, the sky would have been much darker at night than it is today, Sirius would have been a spectacular sight. Ancient civilisations were more in tune with nature and  the predictable changes in the night sky over the year and the re-appearance of the brightest star in the early morning sky on the same date each year would have been significant – whereas for most people today it is unnoticed. The ancient Romans called the month or so of sultry hot weather following the first appearance of Sirius in the morning sky  dies caniculares which translates as Dog Days.

Gradual change in the rising and setting time of Sirius over the centuries

Precession

 As the Earth spins on its axis, two points in the sky known as the North Celestial Pole and the South Celestial Pole remain fixed in the sky, and all celestial objects appear to rotate around them.

The Celestial Poles aren’t completely fixed but move slowly at the rate of around  20.4 seconds of arc, roughly one hundredth the apparent diameter of the Moon in the sky, each year due to an effect known as precession. Today, the North Celestial Pole lies near the bright star Polaris in the constellation Ursa Minor (the Little Bear). Precession causes the coordinates of stars  to slowly change over time and if we go back in time to the year 100 CE  Polaris was about 11 degrees away from the North Celestial Pole and there was no bright Northern Pole Star.

All the stars in the Milky Way are in slightly different orbits around its centre. So, the stars gradually move with respect to each other. This causes Sirius to move  1.33 arcseconds a year compared to the background of much more distant stars. This movement is called proper motion. At this rate it takes 1400 years to move a distance in the sky equal to the apparent diameter of the Moon.

Proper motion

Taking these two effects into account, in the year 100 CE the day Sirius and the Sun rose at the same time in Rome was July 19,  15 days earlier than it is today. So the Dog Days of summer would have been earlier too.

The Brightest Star

Sirius hasn’t always been the brightest star. The distance of stars from the Sun and thus their apparent brightness gradually changes. Sirius is approaching the Solar System at a speed of 5.5 km/s. In   60 000 years time, it will be at its closest to the Sun, 7.8 light years , at which point it will appear 20% brighter than it is today. After this it will recede from the Sun and in millions of years’ time it will be so faint it won’t be visible to the naked eye.

For most of the last 4 million years the giant star Canopus which is 10 700 times more luminous than the Sun but lies 310 light years away has been the brightest star.

Relative sizes of The Sun, Sirius and Canopus

 Other stars (such as Sirius) have appeared brighter during temporary periods, when they have been passing the Solar System at a much closer distance than Canopus. Around the year 200 000 Vega will overtake Sirius as the brightest star; a position it will hold for around 300 000 years. After this for the next half million years Canopus will once again become the brightest star.

Sirius B

In 1862 the American Astronomer Alvan Graham Clark (1832 -1897) discovered that Sirius had a faint  companion star. This companion is now called Sirius B, the main star is now called Sirius A. In visible light, Sirius B is 10000 times fainter than Sirius A. By measuring the wobble of Sirius A, caused by the gravitational effects of Sirius B,  astronomers were able to estimate the mass of Sirius B as 1.02 times the mass of the Sun. By looking at its spectrum, its surface temperature was estimated as 25 000 K. To be so faint it must be very small, its diameter is estimated as only 11 650 km which is less than the Earth’s diameter.

Sirius B was the first white dwarf to be discovered. A white dwarf is the hot dense remnant of a star which has exhausted its nuclear fuel and is slowly cooling. The vast majority of stars in the Milky Way including the Sun are destined to become white dwarfs after they have used up their nuclear fuel.

And Finally – Gliese 710

Recent measurements have shown that the unremarkable  star Gliese 710 which has  60% of the luminosity of the Sun and lies at a distance of 62 light years is moving towards the Solar System at a speed of 14.5 km/s. In 1.3 million years’ time it  is predicted to be 0.17  light years from the Sun. It will become the brightest star and will be three times brighter than Sirius is today – comparable in brightness to Mars and Jupiter. I will talk about Gliese 710 in a future post.

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