What qualifies as work on Sunday?

This answer is additional information to help you with determining conviction:

Our bible study is walking through the Torah at this time, and a helpful portion in determining the heart behind Sabbath for us were these verses in Numbers:

25 And on the seventh day you shall have a holy convocation. You shall not do any ordinary work. 26 "On the day of the firstfruits, when you offer a grain offering of new grain to the LORD at your Feast of Weeks, you shall have a holy convocation. You shall not do any ordinary work... Numbers 28 ESV (italics mine)

The AMP uses the word "servile" instead, which points more to physical labor.

I think what's important for us to understand is that God meant that rest is mandatory. To be honest which day is the Sabbath? That itself is confusing. In some countries the week begins on a Monday and some on a Sunday. I think that God is trying to say that rest is something he invented so that we don't get burned out. I don't think the day and whether we work on that particular day matters. I believe that the point is that rest is necessary and its always good to set apart a day for the lord. We get so caught up in our work, we should set apart one day and focus on him and that's what he meant by keep it holy I guess. I could be wrong.

When I was a member of conservative Baptist churches, the emphasis was on what was allowed, rather than on what wasn't. The mindset was that the Sabbath was obsolete, Sunday was the Lord's Day, and everything done in it should primarily focus on him.

Attending church was okay, including all the work involved in that activity:

• Bathing, dressing, and transporting yourself
• Bathing, dressing, and transporting dependent children
• Preparing lessons (for teachers/preachers)
• Teaching and preaching

Church related work by staff and volunteers was okay, including (but not limited to):

• Opening the building
• Driving a bus/van
• Policing the parking lots
• Greeting regulars and visitors
• Cleaning the building between services
• Running the sound system
• Providing child care/nursery services

Basic activities such as eating were okay, including:

• Preparing your own food
• Preparing food for others
• "Eating out" or visiting a restaurant, which necessarily requires someone else to work

Attending church-related functions was okay, including:

• Social events
• Seminars
• Choir practice
• Prayer meetings
• Bible studies

In other words, a lot of work could be performed. The only things that were really frowned upon were secular jobs, and even those were often considered okay as long as:

• The worker had a family to feed.
• The preacher was in a good mood, or hadn't prepared a message specifically about that person.

The one thing that did not happen (at least among committed church members) was rest. Physical rest apparently became obsolete with the Sabbath, and was replaced with concepts such as "rest in the Lord" or "rest from the world."

Christians are no longer held to the strict laws of Sabbath (an argument can actually be made that Jews were never held to this strict law, either, but that they made it stricter than necessary--but that's beside the point).

I have had jobs in the past where I was required to work on Sunday. To ensure that I still had a "Sabbath day" (day of rest), I chose some other day of the week as my rest day. I think this practice is well within the intention of Sabbath rest, as outlined in the Bible.

But you're asking specifically about a certain task, whether it is work or not. I think Jesus demonstrated that the standard for Sabbath day activity should not be "work,"--as he did several acts of "work" (including harvesting grain, and healing) on the Sabbath. Rather, I think the more important principle is "Remember the Sabbath day and keep it Holy"--that is, use the day for God's glory.

If, for you, that means no studying, then don't study. If for you, studying includes attending a lecture, then don't attend that lecture.

For many people, studying is very stressful, and distracting... for those people, I would think choosing to avoid study as part of their Sabbath ritual is a good idea. For others, studying may be a relaxing thing. I'm not in school, but I find it relaxing to read--and I rarely ever read fiction. That means that almost all of my reading is "study" of some form. If I therefore never "studied" on Sabbath, I would be avoiding one of the most relaxing activities I know.

What it boils down to, I think, is your own personal conviction.

You did the right thing to refuse the lecture, which would have gone against your convictions. If you decide that your convictions should change, I'd say you're also doing the right thing, as there is no "right" answer in this case.