The Herschel 400

For more information go here:
William’s biography https://en.wikipedia.org/wiki/William_Herschel
The Herschel 400 list http://www.saguaroastro.org/content/Herschel-400-objects.htm

Early Life
William Herschel was born in Hanover in 1738 at a time when the English and Hanoverian thrones were united (Initially under George I). He trained as a military musician but eventually ended up in England. There, he worked as a musician and successful composer (CD or MP3 here and also here) based in fashionable Bath. He was also intensely interested in mathematics and astronomy – the general quality of telescopes was poor at the time so he built his own reflector scopes – the telescopes he built were of superb quality and much better than anything else available.

By May 1773 he was actively observing the sky and had become friendly with several eminent “Men of Science”. He was particularly interested in catelogueing interesting findings such as double stars.

Uranus
In 1781 he gained lasting fame by discovering a new planet (with his 7 foot long reflector telescope) – eventually called Uranus (named after the father of Saturn and grandfather of Jupiter) . This was quite an achievement at a time when the “classical” view of the solar system still held sway.

DSO Survey
In recognition of this achievement, he was appointed “The King’s Astronomer” in 1782 and with Royal funding built several new telescopes (including 20 and 40 foot models) at his new home in Windsor (51 North) and undertook one of the first systematic surveys of “deep sky objects (DSOs)” – ie – non-stellar objects. Over the next few years, he systematically classified several thousand DSOs (2508 in total) observed from the Northern hemisphere. His classification system was entirely observational – people didnt know the true nature of galaxies, open clusters, globulars etc. Herschel was aware of Messier’s work and for the most part avoided including his objects.

Herschel’s empirical observation system was as follows:

Class I. Bright Nebulae (288 0bjects)
Class II. Faint Nebulae (909 objects)
Class III. Very Faint Nebulae (984 objects)
Class IV. Planetary Nebulae (78 objects)
Class V. Very Large Nebulae (52 objects)
Class VI. Very Compressed and Rich Clusters of Stars (42)
Class VII. Compressed Clusters of Small (Faint) and Large (Bright) Stars (67)
Class VIII. Coarsely Scattered Clusters of Stars (88)

Total = 2508, 1893 are “Faint” or “Very Faint”

Its interesting to note that Herschel was aware that light took time to travel to his scope and that not only was he looking into the deep distance of space but he was also looking back in time.
People of that era talked about “the fixt stars” and imagined that beyond the solar system, the stars and nebulae (initially thought to be left over bits of star formation) were at much the same distance but Herschel published several papers suggesting that many of the DSOs he observed were very distant.

He spoke of discovering “about 1500 universes” by which he meant galaxies and even drew a primitive map of our Galaxy based on the estimated distance to stars.

Herschel’s Galactic Map

Herschel constructed his map by assuming that all stars were equally bright (A somewhat flawed concept). The brightest star in the night sky is Sirius, so he assigned that a value of 1 distance unit away – He called this unit the Siriometer.

A star that was 1/4 as bright as Sirius must therefore be 16 Siriometers away (inverse square law). In this fashion, his map was constructed.

The Siriometer was a relative scale – there was no conversion factor to a terrestrial distance unit. His calculations yielded the following, which certainly does look a bit like a galaxy:

Herschel_MilkyWay

Planetary Nebulae
He often saw objects that followed stellar motion in the sky but in size and appearance looked more like faint versions of Uranus or Saturn – he called these objects “Planetary Nebulae”. We now know that these objects represent glowing gas shells ejected from a dying star, lit up by UV radiation from the remnant white dwarf within – a sort of cosmic Chinese lantern. These are some of the prettiest objects in the night sky.

720px-NGC_6369HST

This is a beautiful Hubble Space Telescope image of H IV.11 (NGC 6369, “The Little Ghost” nebula) in Ophiuchus. A modern day amateur is more likely to see something like this:

NGC 6369

H IV.11 or if please you, gentle reader, NGC 6369. DSS image. 15 arcminutes across.

Caroline Herschel
His sister Caroline lived with him and often helped his astronomical research by catelogueing his findings. William encouraged her to make her own observations and built special “sweeper” scopes for her, optimised for locating comets. Before long, she was making her own original findings – she discovered 11 DSOs and 8 comets. She published some of these in her own name in the Philosophical Transactions of the Royal Society of London – one of the first females to publish a scientific letter as a solo author. She was awarded a Royal stipend for her contribution to science.

Carolines Rose

This is H VI.30, ie the 30th object in class 6 (NGC 7789) – “Very compressed and rich clusters of stars”. Caroline discovered this open cluster in Cassiopeia in 1783, and apart from H VI.30, it is better known as “Caroline’s Rose”. DSS image – 42 arcminutes across.

This is my favourite quote from Caroline, I think of it every time my scope shows the blur of a brick wall or roof tiles rather than nicely focused stars:

“And of the heavens is no getting, for the high roofs of the opposite houses”.

The Herschel Legacy and the NGC
William’s son, John, was also an eminent mathematician and astronomer and extended his father’s work by catelogueing DSOs in the Southern hemisphere. Whilst in South Africa, he not only performed astronomical observations but with the help of his wife, catelogued and photographed (camera lucida) local flora and fauna – Charles Darwin was influenced in his scientific methods by one of John’s books. He was also a good friend of Charles Babbage, one of the founding fathers of computing. In addition, he was also an early pioneer of photography and invented Cyanotype – an early photographic technique- truly a polymath!

In 1864, John published his “General Catelogue of Nebulae” in the Philosophical Transactions of the Royal Society of London. It comprised over 5000 DSOs, most of these had been directly observed and documented by John, William or Caroline.

Eventually John Dreyer took the GC and other Victorian astronomer’s catelogues and produced the New General Catalogue of 7840 objects – the NGC is still widely used to this day along with a further addendum – the Index Catelogues (IC) of an additional 5386 objects.

The Herschel “400”
From April 1976 onwards, the astronomer James Mullaney published a series of letters and articles in Sky and Telescope and Astronomy magazines suggesting that William Herschel’s 617 objects in Classes I, IV, V, VI, VII and VIII would make ideal DSOs for amateur astronomers looking to go beyond the Messier list. Mullaney suggested that the “Faint” and “Very Faint” objects of Classes II and III could be discarded.

The suggestion was taken up by the Astronomical League – a federation of American astronomy clubs – they eventually decided on a target list of 400 objects drawn from all classes known as the Herschel 400.

I plan to work my way through the “400” but am interested in James Mullaney’s suggestion of fully exploring Classes I, IV, V, VI, VII and VIII.

To date, April 2014, I have observed or imaged 102 Herschel 400 objects.

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s