This is an official photo of a Navy Rear Admiral (lower half):
His paygrade is O-7 (1 star), the equivalent rank in the Army is Brigadier General.
This is an official photo of a Navy Rear Admiral (upper half):
His paygrade is O-8 (2 stars), the equivalent Army rank is Major General.
In practice, the terms "upper half" and "lower half" are dropped.
Now you might be thinking of asking this question: "Comrade Misfit, why do two different naval ranks use the same title?"
Good question, and of course, there is a story behind that.
Historically, 1-star officers in the Navy had the rank of "Commodore." ("Commodore" is also the title of an officer who commands a squadron of ships, but that's not important right now.) For a long time, the Navy did not have 1-star officers; the rank was activated only when there were large wartime fleets. Officers who were promoted from paygrade O-6 to O-7 would don the uniform and insignia of a O-8. "Upper half" and "lower half" were used only for seniority and pay purposes.
What this meant was that on the day when a Captain was promoted to Rear Admiral, he would go into work wearing the uniform of an O-6 and go home wearing the uniform of an O-8.
As you might imagine, that griped the living shit out of the generals in the Army, Marine Corps and, most of all, the Junior Birdmen. The one-star generals in joint commands really hated the fact that they would go from being called "sir" by a naval Captain to having to salute the same guy after his promotion. They resented the hell out of that and complained to the Navy for decades. The Navy, of course, didn't give a flying fuck what the other services thought and regarded the offended brigadier generals as a bunch of crybabies.
In 1980, a bill came up to Congress to require the same treatment of officers by all of the services. The bill, the Defense Officers Personnel Management Act, or "DOPMA," forced the Navy to resurrect the one-star rank, beginning in 1982. Which the Navy did. The Captains who were promoted that year were duly promoted to the resurrected rank of Commodore; they wore one broad stripe on their dress blues.
And in short order, the whining came from the Commodores. The abbreviation for Commodore was "COMO." The Commodores complained that their mail ws being misrouted to the Officers Club (formally known as the "Commissioned Officers Mess, Open, or "COM,O") or to the base communications officer ("COMMO"). The biggest insult came when a four-star Army general, in discussing the replacement naval officer to be assigned to his staff, told the Navy to "send me a real admiral, not one of those weenie commodes."
The whining of the Commodores was written up in the Navy Times newspaper and, in the letters to follow, the junior officers and the enlisted men had a lot of fun at the expense of the Commodores. Most of the letter-writers more or less advised the Commodores to suck it up. One suggested that the ranks of Rear Admiral and Commodore be renamed to "Front Admiral" and "Rear Admiral," respectively, and that the proper halves of a horse costume be issued to those officers. Another published letter noted that Ensigns (pay grade O-1, also sometimes referred to as "enswines" or "butter bars" and equivalent in rank to an Army 2nd lieutenant) also wear a single stripe on their dress uniforms:
and suggested that Commodores be referred to as "Big Ensigns."
The Commodores were, of course, even less amused by the fact that now the rest of the Navy was poking fun at them. More than a couple Commodores had the experience of having junior officers both salute and laugh at them at the same time.
And so, after a decent interval of time, the rank of Commodore became the rank of Rear Admiral.