It is a vowel.

Sometimes, the letter y is a consonant, and other times it is a vowel. The rule for telling the two apart is simple: The letter y is a consonant when it is the first letter of a syllable that has more than one letter. If y is anywhere else in the syllable, it is a vowel.

The theme of the song was used by Mozart as early as 1781 for his piano variations on the French folk song "Ah, vous dirai-je, Maman" which dates to before 1761. This tune is the same used for "Twinkle, Twinkle, Little Star".

The first copyrighted version of the ABC song was published in 1835 by Charles Bradlee of Boston. Its long title was "The A.B.C., a German air with variations for the flute with an easy accompaniment for the piano forte."

"Certified" is cheaper than a "registered letter", which travels outside the regular mail flow, it is signed for and individually handled at every stop in the process. It therefore takes more time to be delivered and is more expensive. Because it is considered more secure, valuables are often sent "registered" rather than "certified." If lost, there is a "log book" record of the travels of the registered letter (or package) at every stop, making recovery more likely.

Certified mail is not insured unless you buy extra insurance from the post office at additional cost. Registered mail is insured up to $25,000, or more with the purchase of additional insurance.

A "certified" letter will be delivered to anyone at a particular address if they sign for it (a secretary at an office, for instance), but a "restricted certified letter" will be delivered only to the person whose name appears on the envelope.

The legal system: courts, lawyers, etc. generally use certified mail with return receipt to prove that their documents were both mailed - and received.

Regular first class mail is the cheapest, followed by "certified", with "registered" being the most expensive.

OBAFGKMN represents the spectral types of stars in order as seen on the Hertzsprung Russell Diagram.

A common mnemonic device for this is: Oh Be A Fine Girl Kiss Me Now.

They stand for the diffrent classes of stars. They are put in these classes depending on what color they are and how hot they are.

The letters refer to how prominently hydrogen is visible in a stars spectrum. It originally existed on a full scale: ABCDEFGHIJKLMNO

As the physics of stars became better understood, many of the letters (CDEL) were rendered obsolete and it was realised that very hot stars have less prominent hydrogen lines, so that the order was re-arranged with O (no lines) then B before AFGKMN.

The reasons for O and B at the top is that in hot stars the Hydrogen becomes ionised which gives a different set of characteristic lines.

As such, they are now in order of overall surface temperature (and color), from white-blue stars more than 33,000K, down to red N class, at less than 3,500K.

Also, as long as we talk about main sequence stars, stars in their main stage of life, it generally indicates size as well, with O class stars being the largest (50+ solar masses) to N being the smallest (0.5 solar masses or less)